[Bug malloc/30579] trim_threshold in realloc lead to high memory usage

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30579

--- Comment #5 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Siddhesh Poyarekar
<siddhesh@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=2fb12bbd092b0c10f1f2083216e723d2406e21c4

commit 2fb12bbd092b0c10f1f2083216e723d2406e21c4
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Thu Jul 6 11:09:44 2023 -0400

    realloc: Limit chunk reuse to only growing requests [BZ #30579]

    The trim_threshold is too aggressive a heuristic to decide if chunk
    reuse is OK for reallocated memory; for repeated small, shrinking
    allocations it leads to internal fragmentation and for repeated larger
    allocations that fragmentation may blow up even worse due to the dynamic
    nature of the threshold.

    Limit reuse only when it is within the alignment padding, which is 2 *
    size_t for heap allocations and a page size for mmapped allocations.
    There's the added wrinkle of THP, but this fix ignores it for now,
    pessimizing that case in favor of keeping fragmentation low.

    This resolves BZ #30579.

    Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
    Reported-by: Nicolas Dusart <nicolas@freedelity.be>
    Reported-by: Aurelien Jarno <aurelien@aurel32.net>
    Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
    Tested-by: Aurelien Jarno <aurelien@aurel32.net>

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