The patch titled
Subject: mm, mmap: limit THP aligment of anonymous mappings to PMD-aligned sizes
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-mmap-limit-thp-aligment-of-anonymous-mappings-to-pmd-aligned-sizes.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mmap-limit-thp-aligment-of-anonymous-mappings-to-pmd-aligned-sizes.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm, mmap: limit THP aligment of anonymous mappings to PMD-aligned sizes
Date: Thu, 24 Oct 2024 17:12:29 +0200
Since commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
boundaries") a mmap() of anonymous memory without a specific address hint
and of at least PMD_SIZE will be aligned to PMD so that it can benefit
from a THP backing page.
However this change has been shown to regress some workloads
significantly. [1] reports regressions in various spec benchmarks, with
up to 600% slowdown of the cactusBSSN benchmark on some platforms. The
benchmark seems to create many mappings of 4632kB, which would have merged
to a large THP-backed area before commit efa7df3e3bb5 and now they are
fragmented to multiple areas each aligned to PMD boundary with gaps
between. The regression then seems to be caused mainly due to the
benchmark's memory access pattern suffering from TLB or cache aliasing due
to the aligned boundaries of the individual areas.
Another known regression bisected to commit efa7df3e3bb5 is darktable [2]
[3] and early testing suggests this patch fixes the regression there as
well.
To fix the regression but still try to benefit from THP-friendly anonymous
mapping alignment, add a condition that the size of the mapping must be a
multiple of PMD size instead of at least PMD size. In case of many
odd-sized mapping like the cactusBSSN creates, those will stop being
aligned and with gaps between, and instead naturally merge again.
Link: https://lkml.kernel.org/r/20241024151228.101841-2-vbabka@xxxxxxx
Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries")
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Reported-by: Michael Matz <matz@xxxxxxx>
Debugged-by: Gabriel Krisman Bertazi <gabriel@xxxxxxxxxx>
Closes: https://bugzilla.suse.com/show_bug.cgi?id=1229012 [1]
Reported-by: Matthias Bodenbinder <matthias@xxxxxxxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219366 [2]
Closes: https://lore.kernel.org/all/2050f0d4-57b0-481d-bab8-05e8d48fed0c@xxxxxxxxxxxxx/ [3]
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Petr Tesarik <ptesarik@xxxxxxxx>
Cc: Thorsten Leemhuis <regressions@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/mm/mmap.c~mm-mmap-limit-thp-aligment-of-anonymous-mappings-to-pmd-aligned-sizes
+++ a/mm/mmap.c
@@ -900,7 +900,8 @@ __get_unmapped_area(struct file *file, u
if (get_area) {
addr = get_area(file, addr, len, pgoff, flags);
- } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
+ } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)
+ && IS_ALIGNED(len, PMD_SIZE)) {
/* Ensures that larger anonymous mappings are THP aligned. */
addr = thp_get_unmapped_area_vmflags(file, addr, len,
pgoff, flags, vm_flags);
_
Patches currently in -mm which might be from vbabka@xxxxxxx are
mm-mmap-limit-thp-aligment-of-anonymous-mappings-to-pmd-aligned-sizes.patch
