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...

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@suse.cz 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@suse.cz Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries") Signed-off-by: Vlastimil Babka vbabka@suse.cz Reported-by: Michael Matz matz@suse.de Debugged-by: Gabriel Krisman Bertazi gabriel@krisman.be Closes: https://bugzilla.suse.com/show_bug.cgi?id=1229012 [1] Reported-by: Matthias Bodenbinder matthias@bodenbinder.de Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219366 [2] Closes: https://lore.kernel.org/all/2050f0d4-57b0-481d-bab8-05e8d48fed0c@leemhuis.in... [3] Cc: Rik van Riel riel@surriel.com Cc: Yang Shi yang@os.amperecomputing.com Cc: Jann Horn jannh@google.com Cc: Liam R. Howlett Liam.Howlett@Oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Petr Tesarik ptesarik@suse.com Cc: Thorsten Leemhuis regressions@leemhuis.info Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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@suse.cz are

mm-mmap-limit-thp-aligment-of-anonymous-mappings-to-pmd-aligned-sizes.patch