From 5f606e928ab08be1825bee3c99cb84f780c2e6e7 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 25 Feb 2025 22:59:22 +0900 Subject: [PATCH] mm, mmap: limit THP alignment of anonymous mappings to PMD-aligned sizes BugLink: https://bugs.launchpad.net/bugs/2099996 [ Upstream commit d4148aeab412432bf928f311eca8a2ba52bb05df ] 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 Reported-by: Michael Matz Debugged-by: Gabriel Krisman Bertazi Closes: https://bugzilla.suse.com/show_bug.cgi?id=1229012 [1] Reported-by: Matthias Bodenbinder Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219366 [2] Closes: https://lore.kernel.org/all/2050f0d4-57b0-481d-bab8-05e8d48fed0c@leemhuis.info/ [3] Reviewed-by: Lorenzo Stoakes Reviewed-by: Yang Shi Cc: Rik van Riel Cc: Jann Horn Cc: Liam R. Howlett Cc: Petr Tesarik Cc: Thorsten Leemhuis Cc: Signed-off-by: Andrew Morton Signed-off-by: Sasha Levin [koichiroden: adjusted context due to missing patch series: "Cover a guard gap corner case" by Rick Edgecombe] Signed-off-by: Koichiro Den Signed-off-by: Stefan Bader --- mm/mmap.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/mmap.c b/mm/mmap.c index e8a497c193b5..ac4e7970e1b2 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1835,7 +1835,8 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len, * so use shmem's get_unmapped_area in case it can be huge. */ get_area = shmem_get_unmapped_area; - } 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. */ get_area = thp_get_unmapped_area; }