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From 98c293c61f770b6b7a22f89a6ea81b711ecb1952 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Fri, 11 Aug 2023 11:18:17 +0200
Subject: [PATCH 06/12] malloc: Enable merging of remainders in memalign (bug
30723)
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>
(cherry picked from commit 542b1105852568c3ebc712225ae78b8c8ba31a78)
---
malloc/malloc.c | 197 +++++++++++++++++++++++++++++-------------------
1 file changed, 121 insertions(+), 76 deletions(-)
diff --git a/malloc/malloc.c b/malloc/malloc.c
index e2f1a615a4..948f9759af 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1086,6 +1086,11 @@ typedef struct malloc_chunk* mchunkptr;
static void* _int_malloc(mstate, size_t);
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,
INTERNAL_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)
__libc_lock_lock (av->mutex);
- nextchunk = chunk_at_offset(p, size);
-
- /* Lightweight tests: check whether the block is already the
- top block. */
- 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)");
-
- 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)");
+ _int_free_merge_chunk (av, p, size);
- free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
+ if (!have_lock)
+ __libc_lock_unlock (av->mutex);
+ }
+ /*
+ If the chunk was allocated via mmap, release via munmap().
+ */
+
+ else {
+ munmap_chunk (p);
+ }
+}
+
+/* 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);
+
+ /* Lightweight tests: check whether the block is already the
+ top block. */
+ 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)) {
- prevsize = prev_size (p);
+ /* Consolidate backward. */
+ if (!prev_inuse(p))
+ {
+ INTERNAL_SIZE_T prevsize = prev_size (p);
size += prevsize;
p = chunk_at_offset(p, -((long) prevsize));
if (__glibc_unlikely (chunksize(p) != prevsize))
@@ -4669,9 +4695,25 @@ _int_free (mstate av, mchunkptr p, int have_lock)
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 */
- nextinuse = inuse_bit_at_offset(nextchunk, nextsize);
+ bool nextinuse = inuse_bit_at_offset (nextchunk, nextsize);
/* consolidate forward */
if (!nextinuse) {
@@ -4686,8 +4728,8 @@ _int_free (mstate av, mchunkptr p, int have_lock)
been given one chance to be used in malloc.
*/
- bck = unsorted_chunks(av);
- fwd = bck->fd;
+ mchunkptr bck = unsorted_chunks (av);
+ mchunkptr fwd = bck->fd;
if (__glibc_unlikely (fwd->bk != bck))
malloc_printerr ("free(): corrupted unsorted chunks");
p->fd = fwd;
@@ -4706,61 +4748,52 @@ _int_free (mstate av, mchunkptr p, int have_lock)
check_free_chunk(av, p);
}
- /*
- If the chunk borders the current high end of memory,
- consolidate into top
- */
-
- else {
+ else
+ {
+ /* If the chunk borders the current high end of memory,
+ consolidate into top. */
size += nextsize;
set_head(p, size | PREV_INUSE);
av->top = p;
check_chunk(av, p);
}
- /*
- 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
- bordering top, so we cannot tell for sure whether threshold
- has 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.
- */
+ return size;
+}
- if ((unsigned long)(size) >= FASTBIN_CONSOLIDATION_THRESHOLD) {
+/* 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. */
+static void
+_int_free_maybe_consolidate (mstate av, INTERNAL_SIZE_T size)
+{
+ /* Unless max_fast is 0, we don't know if there are fastbins
+ bordering top, so we cannot tell for sure whether threshold has
+ 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))
malloc_consolidate(av);
- if (av == &main_arena) {
+ if (av == &main_arena)
+ {
#ifndef MORECORE_CANNOT_TRIM
- if ((unsigned long)(chunksize(av->top)) >=
- (unsigned long)(mp_.trim_threshold))
- systrim(mp_.top_pad, av);
+ if (chunksize (av->top) >= mp_.trim_threshold)
+ systrim (mp_.top_pad, av);
#endif
- } else {
- /* 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));
+ }
+ else
+ {
+ /* 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);
- heap_trim(heap, mp_.top_pad);
- }
+ assert (heap->ar_ptr == av);
+ 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));
set_inuse_bit_at_offset (newp, newsize);
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;
assert (newsize >= nb &&
@@ -5232,15 +5265,27 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
if (!chunk_is_mmapped (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 = chunk_at_offset (p, nb);
- set_head (remainder, remainder_size | PREV_INUSE |
- (av != &main_arena ? NON_MAIN_ARENA : 0));
- set_head_size (p, nb);
- _int_free (av, remainder, 1);
- }
+ if (remainder_size >= MINSIZE
+ || nextchunk == av->top
+ || !inuse_bit_at_offset (nextchunk, nextsize))
+ {
+ /* 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);
--
2.33.0
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