cpython/Objects/obmalloc.c at f0effe637988e936b5a6ece2da789b67723d675c · python/cpython · GitHub

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#include "Python.h"

#ifdef WITH_PYMALLOC

#ifdef HAVE_MMAP

#include <sys/mman.h>

#ifdef MAP_ANONYMOUS

#define ARENAS_USE_MMAP

#endif

#ifdef WITH_VALGRIND

#include <valgrind/valgrind.h>

/* If we're using GCC, use __builtin_expect() to reduce overhead of

the valgrind checks */

#if defined(__GNUC__) && (__GNUC__ > 2) && defined(__OPTIMIZE__)

# define UNLIKELY(value) __builtin_expect((value), 0)

#else

# define UNLIKELY(value) (value)

#endif

/* -1 indicates that we haven't checked that we're running on valgrind yet. */

static int running_on_valgrind = -1;

#endif

/* An object allocator for Python.

Here is an introduction to the layers of the Python memory architecture,

showing where the object allocator is actually used (layer +2), It is

called for every object allocation and deallocation (PyObject_New/Del),

unless the object-specific allocators implement a proprietary allocation

scheme (ex.: ints use a simple free list). This is also the place where

the cyclic garbage collector operates selectively on container objects.

Object-specific allocators

_____ ______ ______ ________

[ int ] [ dict ] [ list ] ... [ string ] Python core |

+3 | <----- Object-specific memory -----> | <-- Non-object memory --> |

_______________________________ | |

[ Python's object allocator ] | |

+2 | ####### Object memory ####### | <------ Internal buffers ------> |

______________________________________________________________ |

[ Python's raw memory allocator (PyMem_ API) ] |

+1 | <----- Python memory (under PyMem manager's control) ------> | |

__________________________________________________________________

[ Underlying general-purpose allocator (ex: C library malloc) ]

0 | <------ Virtual memory allocated for the python process -------> |

=========================================================================

_______________________________________________________________________

[ OS-specific Virtual Memory Manager (VMM) ]

-1 | <--- Kernel dynamic storage allocation & management (page-based) ---> |

__________________________________ __________________________________

[ ] [ ]

-2 | <-- Physical memory: ROM/RAM --> | | <-- Secondary storage (swap) --> |

/*==========================================================================*/

/* A fast, special-purpose memory allocator for small blocks, to be used

on top of a general-purpose malloc -- heavily based on previous art. */

/* Vladimir Marangozov -- August 2000 */

* "Memory management is where the rubber meets the road -- if we do the wrong

* thing at any level, the results will not be good. And if we don't make the

* levels work well together, we are in serious trouble." (1)

* (1) Paul R. Wilson, Mark S. Johnstone, Michael Neely, and David Boles,

* "Dynamic Storage Allocation: A Survey and Critical Review",

* in Proc. 1995 Int'l. Workshop on Memory Management, September 1995.

/* #undef WITH_MEMORY_LIMITS */ /* disable mem limit checks */

/*==========================================================================*/

* Allocation strategy abstract:

* For small requests, the allocator sub-allocates <Big> blocks of memory.

* Requests greater than SMALL_REQUEST_THRESHOLD bytes are routed to the

* system's allocator.

* Small requests are grouped in size classes spaced 8 bytes apart, due

* to the required valid alignment of the returned address. Requests of

* a particular size are serviced from memory pools of 4K (one VMM page).

* Pools are fragmented on demand and contain free lists of blocks of one

* particular size class. In other words, there is a fixed-size allocator

* for each size class. Free pools are shared by the different allocators

* thus minimizing the space reserved for a particular size class.

* This allocation strategy is a variant of what is known as "simple

* segregated storage based on array of free lists". The main drawback of

* simple segregated storage is that we might end up with lot of reserved

* memory for the different free lists, which degenerate in time. To avoid

* this, we partition each free list in pools and we share dynamically the

* reserved space between all free lists. This technique is quite efficient

* for memory intensive programs which allocate mainly small-sized blocks.

* For small requests we have the following table:

* Request in bytes Size of allocated block Size class idx

* ----------------------------------------------------------------

* 1-8 8 0

* 9-16 16 1

* 17-24 24 2

* 25-32 32 3

* 33-40 40 4

* 41-48 48 5

* 49-56 56 6

* 57-64 64 7

* 65-72 72 8

* ... ... ...

* 497-504 504 62

* 505-512 512 63

* 0, SMALL_REQUEST_THRESHOLD + 1 and up: routed to the underlying

* allocator.

/*==========================================================================*/

* -- Main tunable settings section --

* Alignment of addresses returned to the user. 8-bytes alignment works

* on most current architectures (with 32-bit or 64-bit address busses).

* The alignment value is also used for grouping small requests in size

* classes spaced ALIGNMENT bytes apart.

* You shouldn't change this unless you know what you are doing.

#define ALIGNMENT 8 /* must be 2^N */

#define ALIGNMENT_SHIFT 3

#define ALIGNMENT_MASK (ALIGNMENT - 1)

/* Return the number of bytes in size class I, as a uint. */

#define INDEX2SIZE(I) (((uint)(I) + 1) << ALIGNMENT_SHIFT)

* Max size threshold below which malloc requests are considered to be

* small enough in order to use preallocated memory pools. You can tune

* this value according to your application behaviour and memory needs.

* Note: a size threshold of 512 guarantees that newly created dictionaries

* will be allocated from preallocated memory pools on 64-bit.

* The following invariants must hold:

* 1) ALIGNMENT <= SMALL_REQUEST_THRESHOLD <= 512

* 2) SMALL_REQUEST_THRESHOLD is evenly divisible by ALIGNMENT

* Although not required, for better performance and space efficiency,

* it is recommended that SMALL_REQUEST_THRESHOLD is set to a power of 2.

#define SMALL_REQUEST_THRESHOLD 512

#define NB_SMALL_SIZE_CLASSES (SMALL_REQUEST_THRESHOLD / ALIGNMENT)

* The system's VMM page size can be obtained on most unices with a

* getpagesize() call or deduced from various header files. To make

* things simpler, we assume that it is 4K, which is OK for most systems.

* It is probably better if this is the native page size, but it doesn't

* have to be. In theory, if SYSTEM_PAGE_SIZE is larger than the native page

* size, then `POOL_ADDR(p)->arenaindex' could rarely cause a segmentation

* violation fault. 4K is apparently OK for all the platforms that python

* currently targets.

#define SYSTEM_PAGE_SIZE (4 * 1024)

#define SYSTEM_PAGE_SIZE_MASK (SYSTEM_PAGE_SIZE - 1)

* Maximum amount of memory managed by the allocator for small requests.

#ifdef WITH_MEMORY_LIMITS

#ifndef SMALL_MEMORY_LIMIT

#define SMALL_MEMORY_LIMIT (64 * 1024 * 1024) /* 64 MB -- more? */

#endif

* The allocator sub-allocates <Big> blocks of memory (called arenas) aligned

* on a page boundary. This is a reserved virtual address space for the

* current process (obtained through a malloc()/mmap() call). In no way this

* means that the memory arenas will be used entirely. A malloc(<Big>) is

* usually an address range reservation for <Big> bytes, unless all pages within

* this space are referenced subsequently. So malloc'ing big blocks and not

* using them does not mean "wasting memory". It's an addressable range

* wastage...

* Arenas are allocated with mmap() on systems supporting anonymous memory

* mappings to reduce heap fragmentation.

#define ARENA_SIZE (256 << 10) /* 256KB */

#ifdef WITH_MEMORY_LIMITS

#define MAX_ARENAS (SMALL_MEMORY_LIMIT / ARENA_SIZE)

#endif

* Size of the pools used for small blocks. Should be a power of 2,

* between 1K and SYSTEM_PAGE_SIZE, that is: 1k, 2k, 4k.

#define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */

#define POOL_SIZE_MASK SYSTEM_PAGE_SIZE_MASK

* -- End of tunable settings section --

/*==========================================================================*/

* Locking

* To reduce lock contention, it would probably be better to refine the

* crude function locking with per size class locking. I'm not positive

* however, whether it's worth switching to such locking policy because

* of the performance penalty it might introduce.

* The following macros describe the simplest (should also be the fastest)

* lock object on a particular platform and the init/fini/lock/unlock

* operations on it. The locks defined here are not expected to be recursive

* because it is assumed that they will always be called in the order:

* INIT, [LOCK, UNLOCK]*, FINI.

* Python's threads are serialized, so object malloc locking is disabled.

#define SIMPLELOCK_DECL(lock) /* simple lock declaration */

#define SIMPLELOCK_INIT(lock) /* allocate (if needed) and initialize */

#define SIMPLELOCK_FINI(lock) /* free/destroy an existing lock */

#define SIMPLELOCK_LOCK(lock) /* acquire released lock */

#define SIMPLELOCK_UNLOCK(lock) /* release acquired lock */

* Basic types

* I don't care if these are defined in <sys/types.h> or elsewhere. Axiom.

#undef uchar

#define uchar unsigned char /* assuming == 8 bits */

#undef uint

#define uint unsigned int /* assuming >= 16 bits */

#undef ulong

#define ulong unsigned long /* assuming >= 32 bits */

#undef uptr

#define uptr Py_uintptr_t

/* When you say memory, my mind reasons in terms of (pointers to) blocks */

typedef uchar block;

/* Pool for small blocks. */

struct pool_header {

union { block *_padding;

uint count; } ref; /* number of allocated blocks */

block *freeblock; /* pool's free list head */

struct pool_header *nextpool; /* next pool of this size class */

struct pool_header *prevpool; /* previous pool "" */

uint arenaindex; /* index into arenas of base adr */

uint szidx; /* block size class index */

uint nextoffset; /* bytes to virgin block */

uint maxnextoffset; /* largest valid nextoffset */

};

typedef struct pool_header *poolp;

/* Record keeping for arenas. */

struct arena_object {

/* The address of the arena, as returned by malloc. Note that 0

* will never be returned by a successful malloc, and is used

* here to mark an arena_object that doesn't correspond to an

* allocated arena.

uptr address;

/* Pool-aligned pointer to the next pool to be carved off. */

block* pool_address;

/* The number of available pools in the arena: free pools + never-

* allocated pools.

uint nfreepools;

/* The total number of pools in the arena, whether or not available. */

uint ntotalpools;

/* Singly-linked list of available pools. */

struct pool_header* freepools;

/* Whenever this arena_object is not associated with an allocated

* arena, the nextarena member is used to link all unassociated

* arena_objects in the singly-linked `unused_arena_objects` list.

* The prevarena member is unused in this case.

* When this arena_object is associated with an allocated arena

* with at least one available pool, both members are used in the

* doubly-linked `usable_arenas` list, which is maintained in

* increasing order of `nfreepools` values.

* Else this arena_object is associated with an allocated arena

* all of whose pools are in use. `nextarena` and `prevarena`

* are both meaningless in this case.

struct arena_object* nextarena;

struct arena_object* prevarena;

};

#undef ROUNDUP

#define ROUNDUP(x) (((x) + ALIGNMENT_MASK) & ~ALIGNMENT_MASK)

#define POOL_OVERHEAD ROUNDUP(sizeof(struct pool_header))

#define DUMMY_SIZE_IDX 0xffff /* size class of newly cached pools */

/* Round pointer P down to the closest pool-aligned address <= P, as a poolp */

#define POOL_ADDR(P) ((poolp)((uptr)(P) & ~(uptr)POOL_SIZE_MASK))

/* Return total number of blocks in pool of size index I, as a uint. */

#define NUMBLOCKS(I) ((uint)(POOL_SIZE - POOL_OVERHEAD) / INDEX2SIZE(I))

/*==========================================================================*/

* This malloc lock

SIMPLELOCK_DECL(_malloc_lock)

#define LOCK() SIMPLELOCK_LOCK(_malloc_lock)

#define UNLOCK() SIMPLELOCK_UNLOCK(_malloc_lock)

#define LOCK_INIT() SIMPLELOCK_INIT(_malloc_lock)

#define LOCK_FINI() SIMPLELOCK_FINI(_malloc_lock)

* Pool table -- headed, circular, doubly-linked lists of partially used pools.

This is involved. For an index i, usedpools[i+i] is the header for a list of

all partially used pools holding small blocks with "size class idx" i. So

usedpools[0] corresponds to blocks of size 8, usedpools[2] to blocks of size

16, and so on: index 2*i <-> blocks of size (i+1)<<ALIGNMENT_SHIFT.

Pools are carved off an arena's highwater mark (an arena_object's pool_address

member) as needed. Once carved off, a pool is in one of three states forever

after:

used == partially used, neither empty nor full

At least one block in the pool is currently allocated, and at least one

block in the pool is not currently allocated (note this implies a pool

has room for at least two blocks).

This is a pool's initial state, as a pool is created only when malloc

needs space.

The pool holds blocks of a fixed size, and is in the circular list headed

at usedpools[i] (see above). It's linked to the other used pools of the

same size class via the pool_header's nextpool and prevpool members.

If all but one block is currently allocated, a malloc can cause a

transition to the full state. If all but one block is not currently

allocated, a free can cause a transition to the empty state.

full == all the pool's blocks are currently allocated

On transition to full, a pool is unlinked from its usedpools[] list.

It's not linked to from anything then anymore, and its nextpool and

prevpool members are meaningless until it transitions back to used.

A free of a block in a full pool puts the pool back in the used state.

Then it's linked in at the front of the appropriate usedpools[] list, so

that the next allocation for its size class will reuse the freed block.

empty == all the pool's blocks are currently available for allocation

On transition to empty, a pool is unlinked from its usedpools[] list,

and linked to the front of its arena_object's singly-linked freepools list,

via its nextpool member. The prevpool member has no meaning in this case.

Empty pools have no inherent size class: the next time a malloc finds

an empty list in usedpools[], it takes the first pool off of freepools.

If the size class needed happens to be the same as the size class the pool

last had, some pool initialization can be skipped.

Block Management

Blocks within pools are again carved out as needed. pool->freeblock points to

the start of a singly-linked list of free blocks within the pool. When a

block is freed, it's inserted at the front of its pool's freeblock list. Note

that the available blocks in a pool are *not* linked all together when a pool

is initialized. Instead only "the first two" (lowest addresses) blocks are

set up, returning the first such block, and setting pool->freeblock to a

one-block list holding the second such block. This is consistent with that

pymalloc strives at all levels (arena, pool, and block) never to touch a piece

of memory until it's actually needed.

So long as a pool is in the used state, we're certain there *is* a block

available for allocating, and pool->freeblock is not NULL. If pool->freeblock

points to the end of the free list before we've carved the entire pool into

blocks, that means we simply haven't yet gotten to one of the higher-address

blocks. The offset from the pool_header to the start of "the next" virgin

block is stored in the pool_header nextoffset member, and the largest value

of nextoffset that makes sense is stored in the maxnextoffset member when a

pool is initialized. All the blocks in a pool have been passed out at least

once when and only when nextoffset > maxnextoffset.

Major obscurity: While the usedpools vector is declared to have poolp

entries, it doesn't really. It really contains two pointers per (conceptual)

poolp entry, the nextpool and prevpool members of a pool_header. The

excruciating initialization code below fools C so that

usedpool[i+i]

"acts like" a genuine poolp, but only so long as you only reference its

nextpool and prevpool members. The "- 2*sizeof(block *)" gibberish is

compensating for that a pool_header's nextpool and prevpool members

immediately follow a pool_header's first two members:

union { block *_padding;

uint count; } ref;

block *freeblock;

each of which consume sizeof(block *) bytes. So what usedpools[i+i] really

contains is a fudged-up pointer p such that *if* C believes it's a poolp

pointer, then p->nextpool and p->prevpool are both p (meaning that the headed

circular list is empty).

It's unclear why the usedpools setup is so convoluted. It could be to

minimize the amount of cache required to hold this heavily-referenced table

(which only *needs* the two interpool pointer members of a pool_header). OTOH,

referencing code has to remember to "double the index" and doing so isn't

free, usedpools[0] isn't a strictly legal pointer, and we're crucially relying

on that C doesn't insert any padding anywhere in a pool_header at or before

the prevpool member.

**************************************************************************** */

#define PTA(x) ((poolp )((uchar *)&(usedpools[2*(x)]) - 2*sizeof(block *)))

#define PT(x) PTA(x), PTA(x)

static poolp usedpools[2 * ((NB_SMALL_SIZE_CLASSES + 7) / 8) * 8] = {

PT(0), PT(1), PT(2), PT(3), PT(4), PT(5), PT(6), PT(7)

#if NB_SMALL_SIZE_CLASSES > 8

, PT(8), PT(9), PT(10), PT(11), PT(12), PT(13), PT(14), PT(15)

#if NB_SMALL_SIZE_CLASSES > 16

, PT(16), PT(17), PT(18), PT(19), PT(20), PT(21), PT(22), PT(23)

#if NB_SMALL_SIZE_CLASSES > 24

, PT(24), PT(25), PT(26), PT(27), PT(28), PT(29), PT(30), PT(31)

#if NB_SMALL_SIZE_CLASSES > 32

, PT(32), PT(33), PT(34), PT(35), PT(36), PT(37), PT(38), PT(39)

#if NB_SMALL_SIZE_CLASSES > 40

, PT(40), PT(41), PT(42), PT(43), PT(44), PT(45), PT(46), PT(47)

#if NB_SMALL_SIZE_CLASSES > 48

, PT(48), PT(49), PT(50), PT(51), PT(52), PT(53), PT(54), PT(55)

#if NB_SMALL_SIZE_CLASSES > 56

, PT(56), PT(57), PT(58), PT(59), PT(60), PT(61), PT(62), PT(63)

#if NB_SMALL_SIZE_CLASSES > 64

#error "NB_SMALL_SIZE_CLASSES should be less than 64"

#endif /* NB_SMALL_SIZE_CLASSES > 64 */

#endif /* NB_SMALL_SIZE_CLASSES > 56 */

#endif /* NB_SMALL_SIZE_CLASSES > 48 */

#endif /* NB_SMALL_SIZE_CLASSES > 40 */

#endif /* NB_SMALL_SIZE_CLASSES > 32 */

#endif /* NB_SMALL_SIZE_CLASSES > 24 */

#endif /* NB_SMALL_SIZE_CLASSES > 16 */

#endif /* NB_SMALL_SIZE_CLASSES > 8 */

};

/*==========================================================================

Arena management.

`arenas` is a vector of arena_objects. It contains maxarenas entries, some of

which may not be currently used (== they're arena_objects that aren't

currently associated with an allocated arena). Note that arenas proper are

separately malloc'ed.

Prior to Python 2.5, arenas were never free()'ed. Starting with Python 2.5,

we do try to free() arenas, and use some mild heuristic strategies to increase

the likelihood that arenas eventually can be freed.

unused_arena_objects

This is a singly-linked list of the arena_objects that are currently not

being used (no arena is associated with them). Objects are taken off the

head of the list in new_arena(), and are pushed on the head of the list in

PyObject_Free() when the arena is empty. Key invariant: an arena_object

is on this list if and only if its .address member is 0.

usable_arenas

This is a doubly-linked list of the arena_objects associated with arenas

that have pools available. These pools are either waiting to be reused,

or have not been used before. The list is sorted to have the most-

allocated arenas first (ascending order based on the nfreepools member).

This means that the next allocation will come from a heavily used arena,

which gives the nearly empty arenas a chance to be returned to the system.

In my unscientific tests this dramatically improved the number of arenas

that could be freed.

Note that an arena_object associated with an arena all of whose pools are

currently in use isn't on either list.

/* Array of objects used to track chunks of memory (arenas). */

static struct arena_object* arenas = NULL;

/* Number of slots currently allocated in the `arenas` vector. */

static uint maxarenas = 0;

/* The head of the singly-linked, NULL-terminated list of available

* arena_objects.

static struct arena_object* unused_arena_objects = NULL;

/* The head of the doubly-linked, NULL-terminated at each end, list of

* arena_objects associated with arenas that have pools available.

static struct arena_object* usable_arenas = NULL;

/* How many arena_objects do we initially allocate?

* 16 = can allocate 16 arenas = 16 * ARENA_SIZE = 4MB before growing the

* `arenas` vector.

#define INITIAL_ARENA_OBJECTS 16

/* Number of arenas allocated that haven't been free()'d. */

static size_t narenas_currently_allocated = 0;

#ifdef PYMALLOC_DEBUG

/* Total number of times malloc() called to allocate an arena. */

static size_t ntimes_arena_allocated = 0;

/* High water mark (max value ever seen) for narenas_currently_allocated. */

static size_t narenas_highwater = 0;

#endif

/* Allocate a new arena. If we run out of memory, return NULL. Else

* allocate a new arena, and return the address of an arena_object

* describing the new arena. It's expected that the caller will set

* `usable_arenas` to the return value.

static struct arena_object*

new_arena(void)

{

struct arena_object* arenaobj;

uint excess; /* number of bytes above pool alignment */

#ifdef PYMALLOC_DEBUG

if (Py_GETENV("PYTHONMALLOCSTATS"))

_PyObject_DebugMallocStats();

#endif

if (unused_arena_objects == NULL) {

uint i;

uint numarenas;

size_t nbytes;

/* Double the number of arena objects on each allocation.

* Note that it's possible for `numarenas` to overflow.

numarenas = maxarenas ? maxarenas << 1 : INITIAL_ARENA_OBJECTS;

if (numarenas <= maxarenas)

return NULL; /* overflow */

#if SIZEOF_SIZE_T <= SIZEOF_INT

if (numarenas > PY_SIZE_MAX / sizeof(*arenas))

return NULL; /* overflow */

#endif

nbytes = numarenas * sizeof(*arenas);

arenaobj = (struct arena_object *)realloc(arenas, nbytes);

if (arenaobj == NULL)

return NULL;

arenas = arenaobj;

/* We might need to fix pointers that were copied. However,

* new_arena only gets called when all the pages in the

* previous arenas are full. Thus, there are *no* pointers

* into the old array. Thus, we don't have to worry about

* invalid pointers. Just to be sure, some asserts:

assert(usable_arenas == NULL);

assert(unused_arena_objects == NULL);

/* Put the new arenas on the unused_arena_objects list. */

for (i = maxarenas; i < numarenas; ++i) {

arenas[i].address = 0; /* mark as unassociated */

arenas[i].nextarena = i < numarenas - 1 ?

&arenas[i+1] : NULL;

}

/* Update globals. */

unused_arena_objects = &arenas[maxarenas];

maxarenas = numarenas;

}

/* Take the next available arena object off the head of the list. */

assert(unused_arena_objects != NULL);

arenaobj = unused_arena_objects;

unused_arena_objects = arenaobj->nextarena;

assert(arenaobj->address == 0);

#ifdef ARENAS_USE_MMAP

arenaobj->address = (uptr)mmap(NULL, ARENA_SIZE, PROT_READ|PROT_WRITE,

MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

#else

arenaobj->address = (uptr)malloc(ARENA_SIZE);

#endif

if (arenaobj->address == 0) {

/* The allocation failed: return NULL after putting the

* arenaobj back.

arenaobj->nextarena = unused_arena_objects;

unused_arena_objects = arenaobj;

return NULL;

}

++narenas_currently_allocated;

#ifdef PYMALLOC_DEBUG

++ntimes_arena_allocated;

if (narenas_currently_allocated > narenas_highwater)

narenas_highwater = narenas_currently_allocated;

#endif

arenaobj->freepools = NULL;

/* pool_address <- first pool-aligned address in the arena

nfreepools <- number of whole pools that fit after alignment */

arenaobj->pool_address = (block*)arenaobj->address;

arenaobj->nfreepools = ARENA_SIZE / POOL_SIZE;

assert(POOL_SIZE * arenaobj->nfreepools == ARENA_SIZE);

excess = (uint)(arenaobj->address & POOL_SIZE_MASK);

if (excess != 0) {

--arenaobj->nfreepools;

arenaobj->pool_address += POOL_SIZE - excess;

}

arenaobj->ntotalpools = arenaobj->nfreepools;

return arenaobj;

}

Py_ADDRESS_IN_RANGE(P, POOL)

Return true if and only if P is an address that was allocated by pymalloc.

POOL must be the pool address associated with P, i.e., POOL = POOL_ADDR(P)

(the caller is asked to compute this because the macro expands POOL more than

once, and for efficiency it's best for the caller to assign POOL_ADDR(P) to a

variable and pass the latter to the macro; because Py_ADDRESS_IN_RANGE is

called on every alloc/realloc/free, micro-efficiency is important here).

Tricky: Let B be the arena base address associated with the pool, B =

arenas[(POOL)->arenaindex].address. Then P belongs to the arena if and only if

B <= P < B + ARENA_SIZE

Subtracting B throughout, this is true iff

0 <= P-B < ARENA_SIZE

By using unsigned arithmetic, the "0 <=" half of the test can be skipped.

Obscure: A PyMem "free memory" function can call the pymalloc free or realloc

before the first arena has been allocated. `arenas` is still NULL in that

case. We're relying on that maxarenas is also 0 in that case, so that

(POOL)->arenaindex < maxarenas must be false, saving us from trying to index

into a NULL arenas.

Details: given P and POOL, the arena_object corresponding to P is AO =

arenas[(POOL)->arenaindex]. Suppose obmalloc controls P. Then (barring wild

stores, etc), POOL is the correct address of P's pool, AO.address is the

correct base address of the pool's arena, and P must be within ARENA_SIZE of

AO.address. In addition, AO.address is not 0 (no arena can start at address 0

(NULL)). Therefore Py_ADDRESS_IN_RANGE correctly reports that obmalloc

controls P.

Now suppose obmalloc does not control P (e.g., P was obtained via a direct

call to the system malloc() or realloc()). (POOL)->arenaindex may be anything

in this case -- it may even be uninitialized trash. If the trash arenaindex

is >= maxarenas, the macro correctly concludes at once that obmalloc doesn't

control P.

Else arenaindex is < maxarena, and AO is read up. If AO corresponds to an

allocated arena, obmalloc controls all the memory in slice AO.address :

AO.address+ARENA_SIZE. By case assumption, P is not controlled by obmalloc,

so P doesn't lie in that slice, so the macro correctly reports that P is not

controlled by obmalloc.

Finally, if P is not controlled by obmalloc and AO corresponds to an unused

arena_object (one not currently associated with an allocated arena),

AO.address is 0, and the second test in the macro reduces to:

P < ARENA_SIZE

If P >= ARENA_SIZE (extremely likely), the macro again correctly concludes

that P is not controlled by obmalloc. However, if P < ARENA_SIZE, this part

of the test still passes, and the third clause (AO.address != 0) is necessary

to get the correct result: AO.address is 0 in this case, so the macro

correctly reports that P is not controlled by obmalloc (despite that P lies in

slice AO.address : AO.address + ARENA_SIZE).

Note: The third (AO.address != 0) clause was added in Python 2.5. Before

2.5, arenas were never free()'ed, and an arenaindex < maxarena always

corresponded to a currently-allocated arena, so the "P is not controlled by

obmalloc, AO corresponds to an unused arena_object, and P < ARENA_SIZE" case

was impossible.

Note that the logic is excruciating, and reading up possibly uninitialized

memory when P is not controlled by obmalloc (to get at (POOL)->arenaindex)

creates problems for some memory debuggers. The overwhelming advantage is

that this test determines whether an arbitrary address is controlled by

obmalloc in a small constant time, independent of the number of arenas

obmalloc controls. Since this test is needed at every entry point, it's

extremely desirable that it be this fast.

Since Py_ADDRESS_IN_RANGE may be reading from memory which was not allocated

by Python, it is important that (POOL)->arenaindex is read only once, as

another thread may be concurrently modifying the value without holding the

GIL. To accomplish this, the arenaindex_temp variable is used to store

(POOL)->arenaindex for the duration of the Py_ADDRESS_IN_RANGE macro's

execution. The caller of the macro is responsible for declaring this

variable.

#define Py_ADDRESS_IN_RANGE(P, POOL) \

((arenaindex_temp = (POOL)->arenaindex) < maxarenas && \

(uptr)(P) - arenas[arenaindex_temp].address < (uptr)ARENA_SIZE && \

arenas[arenaindex_temp].address != 0)

/* This is only useful when running memory debuggers such as

* Purify or Valgrind. Uncomment to use.

#define Py_USING_MEMORY_DEBUGGER

#ifdef Py_USING_MEMORY_DEBUGGER

/* Py_ADDRESS_IN_RANGE may access uninitialized memory by design

* This leads to thousands of spurious warnings when using

* Purify or Valgrind. By making a function, we can easily

* suppress the uninitialized memory reads in this one function.

* So we won't ignore real errors elsewhere.

* Disable the macro and use a function.

#undef Py_ADDRESS_IN_RANGE

#if defined(__GNUC__) && ((__GNUC__ == 3) && (__GNUC_MINOR__ >= 1) || \

(__GNUC__ >= 4))

#define Py_NO_INLINE __attribute__((__noinline__))

#else

#define Py_NO_INLINE

#endif

/* Don't make static, to try to ensure this isn't inlined. */

int Py_ADDRESS_IN_RANGE(void *P, poolp pool) Py_NO_INLINE;

#undef Py_NO_INLINE

#endif

/*==========================================================================*/

/* malloc. Note that nbytes==0 tries to return a non-NULL pointer, distinct

* from all other currently live pointers. This may not be possible.

* The basic blocks are ordered by decreasing execution frequency,

* which minimizes the number of jumps in the most common cases,

* improves branching prediction and instruction scheduling (small

* block allocations typically result in a couple of instructions).

* Unless the optimizer reorders everything, being too smart...

#undef PyObject_Malloc

void *

PyObject_Malloc(size_t nbytes)

{

block *bp;

poolp pool;

poolp next;

uint size;

#ifdef WITH_VALGRIND

if (UNLIKELY(running_on_valgrind == -1))

running_on_valgrind = RUNNING_ON_VALGRIND;

if (UNLIKELY(running_on_valgrind))

goto redirect;

#endif

* Limit ourselves to PY_SSIZE_T_MAX bytes to prevent security holes.

* Most python internals blindly use a signed Py_ssize_t to track

* things without checking for overflows or negatives.

* As size_t is unsigned, checking for nbytes < 0 is not required.

if (nbytes > PY_SSIZE_T_MAX)

return NULL;

* This implicitly redirects malloc(0).

if ((nbytes - 1) < SMALL_REQUEST_THRESHOLD) {

LOCK();

* Most frequent paths first

size = (uint)(nbytes - 1) >> ALIGNMENT_SHIFT;

pool = usedpools[size + size];

if (pool != pool->nextpool) {

* There is a used pool for this size class.

* Pick up the head block of its free list.

++pool->ref.count;

bp = pool->freeblock;

assert(bp != NULL);

if ((pool->freeblock = *(block **)bp) != NULL) {

UNLOCK();

return (void *)bp;

}

* Reached the end of the free list, try to extend it.

if (pool->nextoffset <= pool->maxnextoffset) {

/* There is room for another block. */

pool->freeblock = (block*)pool +

pool->nextoffset;

pool->nextoffset += INDEX2SIZE(size);

*(block **)(pool->freeblock) = NULL;

UNLOCK();

return (void *)bp;

}

/* Pool is full, unlink from used pools. */

next = pool->nextpool;

pool = pool->prevpool;

next->prevpool = pool;

pool->nextpool = next;

UNLOCK();

return (void *)bp;

}

/* There isn't a pool of the right size class immediately

* available: use a free pool.

if (usable_arenas == NULL) {

/* No arena has a free pool: allocate a new arena. */

#ifdef WITH_MEMORY_LIMITS

if (narenas_currently_allocated >= MAX_ARENAS) {

UNLOCK();

goto redirect;

}

#endif

usable_arenas = new_arena();

if (usable_arenas == NULL) {

UNLOCK();

goto redirect;

}

usable_arenas->nextarena =

usable_arenas->prevarena = NULL;

}

assert(usable_arenas->address != 0);

/* Try to get a cached free pool. */

pool = usable_arenas->freepools;

if (pool != NULL) {

/* Unlink from cached pools. */

usable_arenas->freepools = pool->nextpool;

/* This arena already had the smallest nfreepools

* value, so decreasing nfreepools doesn't change

* that, and we don't need to rearrange the

* usable_arenas list. However, if the arena has

* become wholly allocated, we need to remove its

* arena_object from usable_arenas.

--usable_arenas->nfreepools;

if (usable_arenas->nfreepools == 0) {

/* Wholly allocated: remove. */

assert(usable_arenas->freepools == NULL);

assert(usable_arenas->nextarena == NULL ||

usable_arenas->nextarena->prevarena ==

usable_arenas);

usable_arenas = usable_arenas->nextarena;

if (usable_arenas != NULL) {

usable_arenas->prevarena = NULL;

assert(usable_arenas->address != 0);

}

else {

/* nfreepools > 0: it must be that freepools

* isn't NULL, or that we haven't yet carved

* off all the arena's pools for the first

* time.

assert(usable_arenas->freepools != NULL ||

usable_arenas->pool_address <=

(block*)usable_arenas->address +

ARENA_SIZE - POOL_SIZE);

}

init_pool:

/* Frontlink to used pools. */

next = usedpools[size + size]; /* == prev */

pool->nextpool = next;

pool->prevpool = next;

next->nextpool = pool;

next->prevpool = pool;

pool->ref.count = 1;

if (pool->szidx == size) {

/* Luckily, this pool last contained blocks

* of the same size class, so its header

* and free list are already initialized.

bp = pool->freeblock;

pool->freeblock = *(block **)bp;

UNLOCK();

return (void *)bp;

}

* Initialize the pool header, set up the free list to

* contain just the second block, and return the first

* block.

pool->szidx = size;

size = INDEX2SIZE(size);

bp = (block *)pool + POOL_OVERHEAD;

pool->nextoffset = POOL_OVERHEAD + (size << 1);

pool->maxnextoffset = POOL_SIZE - size;

pool->freeblock = bp + size;

*(block **)(pool->freeblock) = NULL;

UNLOCK();

return (void *)bp;

}

/* Carve off a new pool. */

assert(usable_arenas->nfreepools > 0);

assert(usable_arenas->freepools == NULL);

pool = (poolp)usable_arenas->pool_address;

assert((block*)pool <= (block*)usable_arenas->address +

ARENA_SIZE - POOL_SIZE);

pool->arenaindex = usable_arenas - arenas;

assert(&arenas[pool->arenaindex] == usable_arenas);

pool->szidx = DUMMY_SIZE_IDX;

usable_arenas->pool_address += POOL_SIZE;

--usable_arenas->nfreepools;

if (usable_arenas->nfreepools == 0) {

assert(usable_arenas->nextarena == NULL ||

usable_arenas->nextarena->prevarena ==

usable_arenas);

/* Unlink the arena: it is completely allocated. */

usable_arenas = usable_arenas->nextarena;

if (usable_arenas != NULL) {

usable_arenas->prevarena = NULL;

assert(usable_arenas->address != 0);

}

goto init_pool;

}

/* The small block allocator ends here. */

redirect:

/* Redirect the original request to the underlying (libc) allocator.

* We jump here on bigger requests, on error in the code above (as a

* last chance to serve the request) or when the max memory limit

* has been reached.

if (nbytes == 0)

nbytes = 1;

return (void *)malloc(nbytes);

}

/* free */

#undef PyObject_Free

void

PyObject_Free(void *p)

{

poolp pool;

block *lastfree;

poolp next, prev;

uint size;

#ifndef Py_USING_MEMORY_DEBUGGER

uint arenaindex_temp;

#endif

if (p == NULL) /* free(NULL) has no effect */

return;

#ifdef WITH_VALGRIND

if (UNLIKELY(running_on_valgrind > 0))

goto redirect;

#endif

pool = POOL_ADDR(p);

if (Py_ADDRESS_IN_RANGE(p, pool)) {

/* We allocated this address. */

LOCK();

/* Link p to the start of the pool's freeblock list. Since

* the pool had at least the p block outstanding, the pool

* wasn't empty (so it's already in a usedpools[] list, or

* was full and is in no list -- it's not in the freeblocks

* list in any case).

assert(pool->ref.count > 0); /* else it was empty */

*(block **)p = lastfree = pool->freeblock;

pool->freeblock = (block *)p;

View remainder of file in raw view

cpython/Objects/obmalloc.c at f0effe637988e936b5a6ece2da789b67723d675c · python/cpython · GitHub

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