forked from pool/glibc
69 lines
2.6 KiB
Diff
69 lines
2.6 KiB
Diff
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From 0930ff8eb35cb493c945f176c3c9ab320f4d1b86 Mon Sep 17 00:00:00 2001
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From: Siddhesh Poyarekar <siddhesh@sourceware.org>
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Date: Thu, 6 Jul 2023 11:09:44 -0400
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Subject: [PATCH] realloc: Limit chunk reuse to only growing requests [BZ
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#30579]
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The trim_threshold is too aggressive a heuristic to decide if chunk
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reuse is OK for reallocated memory; for repeated small, shrinking
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allocations it leads to internal fragmentation and for repeated larger
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allocations that fragmentation may blow up even worse due to the dynamic
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nature of the threshold.
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Limit reuse only when it is within the alignment padding, which is 2 *
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size_t for heap allocations and a page size for mmapped allocations.
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There's the added wrinkle of THP, but this fix ignores it for now,
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pessimizing that case in favor of keeping fragmentation low.
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This resolves BZ #30579.
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Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
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Reported-by: Nicolas Dusart <nicolas@freedelity.be>
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Reported-by: Aurelien Jarno <aurelien@aurel32.net>
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Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
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Tested-by: Aurelien Jarno <aurelien@aurel32.net>
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(cherry picked from commit 2fb12bbd092b0c10f1f2083216e723d2406e21c4)
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---
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malloc/malloc.c | 23 +++++++++++++++--------
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1 file changed, 15 insertions(+), 8 deletions(-)
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diff --git a/malloc/malloc.c b/malloc/malloc.c
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index fd8b52bfac..67df9f8c51 100644
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--- a/malloc/malloc.c
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+++ b/malloc/malloc.c
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@@ -3398,16 +3398,23 @@ __libc_realloc (void *oldmem, size_t bytes)
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if (__glibc_unlikely (mtag_enabled))
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*(volatile char*) oldmem;
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- /* Return the chunk as is whenever possible, i.e. there's enough usable space
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- but not so much that we end up fragmenting the block. We use the trim
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- threshold as the heuristic to decide the latter. */
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- size_t usable = musable (oldmem);
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- if (bytes <= usable
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- && (unsigned long) (usable - bytes) <= mp_.trim_threshold)
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- return oldmem;
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-
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/* chunk corresponding to oldmem */
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const mchunkptr oldp = mem2chunk (oldmem);
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+
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+ /* Return the chunk as is if the request grows within usable bytes, typically
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+ into the alignment padding. We want to avoid reusing the block for
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+ shrinkages because it ends up unnecessarily fragmenting the address space.
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+ This is also why the heuristic misses alignment padding for THP for
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+ now. */
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+ size_t usable = musable (oldmem);
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+ if (bytes <= usable)
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+ {
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+ size_t difference = usable - bytes;
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+ if ((unsigned long) difference < 2 * sizeof (INTERNAL_SIZE_T)
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+ || (chunk_is_mmapped (oldp) && difference <= GLRO (dl_pagesize)))
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+ return oldmem;
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+ }
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+
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/* its size */
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const INTERNAL_SIZE_T oldsize = chunksize (oldp);
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--
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2.41.0
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