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ANDROID: 16K: Exclude ELF padding for fault around range

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Userspace apps often analyze memory consumption by the use of mm
rss_stat counters -- via the kmem/rss_stat trace event or from
/proc/<pid>/statm.

rss_stat counters are only updated when the PTEs are updated. What this
means is that pages can be present in the page cache from readahead but
not visible to userspace (not attributed to the app) as there is no
corresponding VMA (PTEs) for the respective page cache pages.

A side effect of the loader now extending ELF LOAD segments to be
contiguously mapped in the virtual address space, means that the VMA is
extended to cover the padding pages.

When filesystems, such as f2fs and ext4, that implement
vm_ops->map_pages() attempt to perform a do_fault_around() the extent of
the fault around is restricted by the area of the enclosing VMA. Since
the loader extends LOAD segment VMAs to be contiguously mapped, the extent
of the fault around is also increased. The result of which, is that the
PTEs corresponding to the padding pages are updated and reflected in the
rss_stat counters.

It is not common that userspace application developers be aware of this
nuance in the kernel's memory accounting. To avoid apparent regressions
in memory usage to userspace, restrict the fault around range to only
valid data pages (i.e. exclude the padding pages at the end of the VMA).

Bug: 330117029
Bug: 327600007
Bug: 330767927
Bug: 328266487
Bug: 329803029
Change-Id: I2c7a39ec1b040be2b9fb47801f95042f5dbf869d
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
This commit is contained in:
Kalesh Singh 2024-04-04 22:37:48 -07:00
parent 778abad3ac
commit abbc0d53ee
1 changed files with 2 additions and 1 deletions
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3
mm/memory.c
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@ -57,6 +57,7 @@
#include <linux/delayacct.h>
#include <linux/init.h>
#include <linux/pfn_t.h>
#include <linux/pgsize_migration.h>
#include <linux/writeback.h>
#include <linux/memcontrol.h>
#include <linux/mmu_notifier.h>
@ -4359,7 +4360,7 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
end_pgoff = start_pgoff -
((address >> PAGE_SHIFT) & (PTRS_PER_PTE - 1)) +
PTRS_PER_PTE - 1;
end_pgoff = min3(end_pgoff, vma_pages(vmf->vma) + vmf->vma->vm_pgoff - 1,
end_pgoff = min3(end_pgoff, vma_data_pages(vmf->vma) + vmf->vma->vm_pgoff - 1,
start_pgoff + nr_pages - 1);
if (!(vmf->flags & FAULT_FLAG_SPECULATIVE) &&