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malloc: Enable merging of remainders in memalign (bug 30723)

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Previously, calling _int_free from _int_memalign could put remainders
into the tcache or into fastbins, where they are invisible to the
low-level allocator.  This results in missed merge opportunities
because once these freed chunks become available to the low-level
allocator, further memalign allocations (even of the same size are)
likely obstructing merges.

Furthermore, during forwards merging in _int_memalign, do not
completely give up when the remainder is too small to serve as a
chunk on its own.  We can still give it back if it can be merged
with the following unused chunk.  This makes it more likely that
memalign calls in a loop achieve a compact memory layout,
independently of initial heap layout.

Drop some useless (unsigned long) casts along the way, and tweak
the style to more closely match GNU on changed lines.

Reviewed-by: DJ Delorie <dj@redhat.com>
This commit is contained in:
Florian Weimer 2023-08-11 11:18:17 +02:00
parent 039ff51ac7
commit 542b110585
1 changed files with 119 additions and 74 deletions
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193
malloc/malloc.c
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@ -1086,6 +1086,11 @@ typedef struct malloc_chunk* mchunkptr;
static void* _int_malloc(mstate, size_t); static void* _int_malloc(mstate, size_t);
static void _int_free(mstate, mchunkptr, int); static void _int_free(mstate, mchunkptr, int);
static void _int_free_merge_chunk (mstate, mchunkptr, INTERNAL_SIZE_T);
static INTERNAL_SIZE_T _int_free_create_chunk (mstate,
mchunkptr, INTERNAL_SIZE_T,
mchunkptr, INTERNAL_SIZE_T);
static void _int_free_maybe_consolidate (mstate, INTERNAL_SIZE_T);
static void* _int_realloc(mstate, mchunkptr, INTERNAL_SIZE_T, static void* _int_realloc(mstate, mchunkptr, INTERNAL_SIZE_T,
INTERNAL_SIZE_T); INTERNAL_SIZE_T);
static void* _int_memalign(mstate, size_t, size_t); static void* _int_memalign(mstate, size_t, size_t);
@ -4637,31 +4642,52 @@ _int_free (mstate av, mchunkptr p, int have_lock)
if (!have_lock) if (!have_lock)
__libc_lock_lock (av->mutex); __libc_lock_lock (av->mutex);
nextchunk = chunk_at_offset(p, size); _int_free_merge_chunk (av, p, size);
/* Lightweight tests: check whether the block is already the if (!have_lock)
top block. */ __libc_lock_unlock (av->mutex);
if (__glibc_unlikely (p == av->top)) }
malloc_printerr ("double free or corruption (top)"); /*
/* Or whether the next chunk is beyond the boundaries of the arena. */ If the chunk was allocated via mmap, release via munmap().
if (__builtin_expect (contiguous (av) */
&& (char *) nextchunk
>= ((char *) av->top + chunksize(av->top)), 0))
malloc_printerr ("double free or corruption (out)");
/* Or whether the block is actually not marked used. */
if (__glibc_unlikely (!prev_inuse(nextchunk)))
malloc_printerr ("double free or corruption (!prev)");
nextsize = chunksize(nextchunk); else {
if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0) munmap_chunk (p);
|| __builtin_expect (nextsize >= av->system_mem, 0)) }
malloc_printerr ("free(): invalid next size (normal)"); }
free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ); /* Try to merge chunk P of SIZE bytes with its neighbors. Put the
resulting chunk on the appropriate bin list. P must not be on a
bin list yet, and it can be in use. */
static void
_int_free_merge_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size)
{
mchunkptr nextchunk = chunk_at_offset(p, size);
/* consolidate backward */ /* Lightweight tests: check whether the block is already the
if (!prev_inuse(p)) { top block. */
prevsize = prev_size (p); if (__glibc_unlikely (p == av->top))
malloc_printerr ("double free or corruption (top)");
/* Or whether the next chunk is beyond the boundaries of the arena. */
if (__builtin_expect (contiguous (av)
&& (char *) nextchunk
>= ((char *) av->top + chunksize(av->top)), 0))
malloc_printerr ("double free or corruption (out)");
/* Or whether the block is actually not marked used. */
if (__glibc_unlikely (!prev_inuse(nextchunk)))
malloc_printerr ("double free or corruption (!prev)");
INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0)
|| __builtin_expect (nextsize >= av->system_mem, 0))
malloc_printerr ("free(): invalid next size (normal)");
free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
/* Consolidate backward. */
if (!prev_inuse(p))
{
INTERNAL_SIZE_T prevsize = prev_size (p);
size += prevsize; size += prevsize;
p = chunk_at_offset(p, -((long) prevsize)); p = chunk_at_offset(p, -((long) prevsize));
if (__glibc_unlikely (chunksize(p) != prevsize)) if (__glibc_unlikely (chunksize(p) != prevsize))
@ -4669,9 +4695,25 @@ _int_free (mstate av, mchunkptr p, int have_lock)
unlink_chunk (av, p); unlink_chunk (av, p);
} }
if (nextchunk != av->top) { /* Write the chunk header, maybe after merging with the following chunk. */
size = _int_free_create_chunk (av, p, size, nextchunk, nextsize);
_int_free_maybe_consolidate (av, size);
}
/* Create a chunk at P of SIZE bytes, with SIZE potentially increased
to cover the immediately following chunk NEXTCHUNK of NEXTSIZE
bytes (if NEXTCHUNK is unused). The chunk at P is not actually
read and does not have to be initialized. After creation, it is
placed on the appropriate bin list. The function returns the size
of the new chunk. */
static INTERNAL_SIZE_T
_int_free_create_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size,
mchunkptr nextchunk, INTERNAL_SIZE_T nextsize)
{
if (nextchunk != av->top)
{
/* get and clear inuse bit */ /* get and clear inuse bit */
nextinuse = inuse_bit_at_offset(nextchunk, nextsize); bool nextinuse = inuse_bit_at_offset (nextchunk, nextsize);
/* consolidate forward */ /* consolidate forward */
if (!nextinuse) { if (!nextinuse) {
@ -4686,8 +4728,8 @@ _int_free (mstate av, mchunkptr p, int have_lock)
been given one chance to be used in malloc. been given one chance to be used in malloc.
*/ */
bck = unsorted_chunks(av); mchunkptr bck = unsorted_chunks (av);
fwd = bck->fd; mchunkptr fwd = bck->fd;
if (__glibc_unlikely (fwd->bk != bck)) if (__glibc_unlikely (fwd->bk != bck))
malloc_printerr ("free(): corrupted unsorted chunks"); malloc_printerr ("free(): corrupted unsorted chunks");
p->fd = fwd; p->fd = fwd;
@ -4706,61 +4748,52 @@ _int_free (mstate av, mchunkptr p, int have_lock)
check_free_chunk(av, p); check_free_chunk(av, p);
} }
/* else
If the chunk borders the current high end of memory, {
consolidate into top /* If the chunk borders the current high end of memory,
*/ consolidate into top. */
else {
size += nextsize; size += nextsize;
set_head(p, size | PREV_INUSE); set_head(p, size | PREV_INUSE);
av->top = p; av->top = p;
check_chunk(av, p); check_chunk(av, p);
} }
/* return size;
If freeing a large space, consolidate possibly-surrounding }
chunks. Then, if the total unused topmost memory exceeds trim
threshold, ask malloc_trim to reduce top.
Unless max_fast is 0, we don't know if there are fastbins /* If freeing a large space, consolidate possibly-surrounding
bordering top, so we cannot tell for sure whether threshold chunks. Then, if the total unused topmost memory exceeds trim
has been reached unless fastbins are consolidated. But we threshold, ask malloc_trim to reduce top. */
don't want to consolidate on each free. As a compromise, static void
consolidation is performed if FASTBIN_CONSOLIDATION_THRESHOLD _int_free_maybe_consolidate (mstate av, INTERNAL_SIZE_T size)
is reached. {
*/ /* Unless max_fast is 0, we don't know if there are fastbins
bordering top, so we cannot tell for sure whether threshold has
if ((unsigned long)(size) >= FASTBIN_CONSOLIDATION_THRESHOLD) { been reached unless fastbins are consolidated. But we don't want
to consolidate on each free. As a compromise, consolidation is
performed if FASTBIN_CONSOLIDATION_THRESHOLD is reached. */
if (size >= FASTBIN_CONSOLIDATION_THRESHOLD)
{
if (atomic_load_relaxed (&av->have_fastchunks)) if (atomic_load_relaxed (&av->have_fastchunks))
malloc_consolidate(av); malloc_consolidate(av);
if (av == &main_arena) { if (av == &main_arena)
{
#ifndef MORECORE_CANNOT_TRIM #ifndef MORECORE_CANNOT_TRIM
if ((unsigned long)(chunksize(av->top)) >= if (chunksize (av->top) >= mp_.trim_threshold)
(unsigned long)(mp_.trim_threshold)) systrim (mp_.top_pad, av);
systrim(mp_.top_pad, av);
#endif #endif
} else { }
/* Always try heap_trim(), even if the top chunk is not else
large, because the corresponding heap might go away. */ {
heap_info *heap = heap_for_ptr(top(av)); /* Always try heap_trim, even if the top chunk is not large,
because the corresponding heap might go away. */
heap_info *heap = heap_for_ptr (top (av));
assert(heap->ar_ptr == av); assert (heap->ar_ptr == av);
heap_trim(heap, mp_.top_pad); heap_trim (heap, mp_.top_pad);
} }
} }
if (!have_lock)
__libc_lock_unlock (av->mutex);
}
/*
If the chunk was allocated via mmap, release via munmap().
*/
else {
munmap_chunk (p);
}
} }
/* /*
@ -5221,7 +5254,7 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
(av != &main_arena ? NON_MAIN_ARENA : 0)); (av != &main_arena ? NON_MAIN_ARENA : 0));
set_inuse_bit_at_offset (newp, newsize); set_inuse_bit_at_offset (newp, newsize);
set_head_size (p, leadsize | (av != &main_arena ? NON_MAIN_ARENA : 0)); set_head_size (p, leadsize | (av != &main_arena ? NON_MAIN_ARENA : 0));
_int_free (av, p, 1); _int_free_merge_chunk (av, p, leadsize);
p = newp; p = newp;
assert (newsize >= nb && assert (newsize >= nb &&
@ -5232,15 +5265,27 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
if (!chunk_is_mmapped (p)) if (!chunk_is_mmapped (p))
{ {
size = chunksize (p); size = chunksize (p);
if ((unsigned long) (size) > (unsigned long) (nb + MINSIZE)) mchunkptr nextchunk = chunk_at_offset(p, size);
INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
if (size > nb)
{ {
remainder_size = size - nb; remainder_size = size - nb;
remainder = chunk_at_offset (p, nb); if (remainder_size >= MINSIZE
set_head (remainder, remainder_size | PREV_INUSE | || nextchunk == av->top
(av != &main_arena ? NON_MAIN_ARENA : 0)); || !inuse_bit_at_offset (nextchunk, nextsize))
set_head_size (p, nb); {
_int_free (av, remainder, 1); /* We can only give back the tail if it is larger than
} MINSIZE, or if the following chunk is unused (top
chunk or unused in-heap chunk). Otherwise we would
create a chunk that is smaller than MINSIZE. */
remainder = chunk_at_offset (p, nb);
set_head_size (p, nb);
remainder_size = _int_free_create_chunk (av, remainder,
remainder_size,
nextchunk, nextsize);
_int_free_maybe_consolidate (av, remainder_size);
}
}
} }
check_inuse_chunk (av, p); check_inuse_chunk (av, p);