The patch titled Subject: mm: thp_get_unmapped_area must honour topdown preference has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-thp_get_unmapped_area-must-honour-topdown-preference.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: Ryan Roberts ryan.roberts@arm.com Subject: mm: thp_get_unmapped_area must honour topdown preference Date: Tue, 23 Jan 2024 17:14:20 +0000

The addition of commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries") caused the "virtual_address_range" mm selftest to start failing on arm64. Let's fix that regression.

There were 2 visible problems when running the test; 1) it takes much longer to execute, and 2) the test fails. Both are related:

The (first part of the) test allocates as many 1GB anonymous blocks as it can in the low 256TB of address space, passing NULL as the addr hint to mmap. Before the faulty patch, all allocations were abutted and contained in a single, merged VMA. However, after this patch, each allocation is in its own VMA, and there is a 2M gap between each VMA. This causes the 2 problems in the test: 1) mmap becomes MUCH slower because there are so many VMAs to check to find a new 1G gap. 2) mmap fails once it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting this limit then causes a subsequent calloc() to fail, which causes the test to fail.

The problem is that arm64 (unlike x86) selects ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area() allocates len+2M then always aligns to the bottom of the discovered gap. That causes the 2M hole.

Fix this by detecting cases where we can still achive the alignment goal when moved to the top of the allocated area, if configured to prefer top-down allocation.

While we are at it, fix thp_get_unmapped_area's use of pgoff, which should always be zero for anonymous mappings. Prior to the faulty change, while it was possible for user space to pass in pgoff!=0, the old mm->get_unmapped_area() handler would not use it. thp_get_unmapped_area() does use it, so let's explicitly zero it before calling the handler. This should also be the correct behavior for arches that define their own get_unmapped_area() handler.

Link: https://lkml.kernel.org/r/20240123171420.3970220-1-ryan.roberts@arm.com Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries") Closes: https://lore.kernel.org/linux-mm/1e8f5ac7-54ce-433a-ae53-81522b2320e1@arm.co... Signed-off-by: Ryan Roberts ryan.roberts@arm.com Reviewed-by: Yang Shi shy828301@gmail.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Rik van Riel riel@surriel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/huge_memory.c | 10 ++++++++-- mm/mmap.c | 6 ++++-- 2 files changed, 12 insertions(+), 4 deletions(-)

--- a/mm/huge_memory.c~mm-thp_get_unmapped_area-must-honour-topdown-preference +++ a/mm/huge_memory.c @@ -810,7 +810,7 @@ static unsigned long __thp_get_unmapped_ { loff_t off_end = off + len; loff_t off_align = round_up(off, size); - unsigned long len_pad, ret; + unsigned long len_pad, ret, off_sub;

if (IS_ENABLED(CONFIG_32BIT) || in_compat_syscall()) return 0; @@ -839,7 +839,13 @@ static unsigned long __thp_get_unmapped_ if (ret == addr) return addr;

- ret += (off - ret) & (size - 1); + off_sub = (off - ret) & (size - 1); + + if (current->mm->get_unmapped_area == arch_get_unmapped_area_topdown && + !off_sub) + return ret + size; + + ret += off_sub; return ret; }

--- a/mm/mmap.c~mm-thp_get_unmapped_area-must-honour-topdown-preference +++ a/mm/mmap.c @@ -1825,15 +1825,17 @@ get_unmapped_area(struct file *file, uns /* * mmap_region() will call shmem_zero_setup() to create a file, * so use shmem's get_unmapped_area in case it can be huge. - * do_mmap() will clear pgoff, so match alignment. */ - pgoff = 0; get_area = shmem_get_unmapped_area; } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) { /* Ensures that larger anonymous mappings are THP aligned. */ get_area = thp_get_unmapped_area; }

+ /* Always treat pgoff as zero for anonymous memory. */ + if (!file) + pgoff = 0; + addr = get_area(file, addr, len, pgoff, flags); if (IS_ERR_VALUE(addr)) return addr; _

Patches currently in -mm which might be from ryan.roberts@arm.com are

selftests-mm-ksm_tests-should-only-madv_hugepage-valid-memory.patch mm-thp_get_unmapped_area-must-honour-topdown-preference.patch tools-mm-add-thpmaps-script-to-dump-thp-usage-info.patch