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/mm-thp_get_unmapped_area-must-honour-topdown-preference.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: Ryan Roberts <ryan.roberts@xxxxxxx>
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@xxxxxxx
Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries")
Closes: https://lore.kernel.org/linux-mm/1e8f5ac7-54ce-433a-ae53-81522b2320e1@xxxxxxx/
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxx 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
