github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-05-30 18:13:41 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

mm: readahead: make thp readahead conditional to mmap_miss logic

Browse Source 
Commit 4687fdbb80 ("mm/filemap: Support VM_HUGEPAGE for file mappings")
introduced a special handling for VM_HUGEPAGE mappings: even if the
readahead is disabled, 1 or 2 HPAGE_PMD_ORDER pages are allocated.

This change causes a significant regression for containers with a tight
memory.max limit, if VM_HUGEPAGE is widely used.  Prior to this commit,
mmap_miss logic would eventually lead to the readahead disablement,
effectively reducing the memory pressure in the cgroup.  With this change
the kernel is trying to allocate 1-2 huge pages for each fault, no matter
if these pages are used or not before being evicted, increasing the memory
pressure multi-fold.

To fix the regression, let's make the new VM_HUGEPAGE conditional to the
mmap_miss check, but keep independent from the ra->ra_pages.  This way the
main intention of commit 4687fdbb80 ("mm/filemap: Support VM_HUGEPAGE
for file mappings") stays intact, but the regression is resolved.

The logic behind this changes is simple: even if a user explicitly
requests using huge pages to back the file mapping (using VM_HUGEPAGE
flag), under a very strong memory pressure it's better to fall back to
ordinary pages.

Link: https://lkml.kernel.org/r/20251006175106.377411-1-roman.gushchin@linux.dev
Fixes: 4687fdbb80 ("mm/filemap: Support VM_HUGEPAGE for file mappings")
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Roman Gushchin 2025-10-06 10:51:06 -07:00 committed by Andrew Morton
parent c0efdb373c
commit 9f1edf1aed
1 changed files with 38 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

68
mm/filemap.c
View File

@ -3253,11 +3253,47 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
struct file *fpin = NULL;
vm_flags_t vm_flags = vmf->vma->vm_flags;
bool force_thp_readahead = false;
unsigned short mmap_miss;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/* Use the readahead code, even if readahead is disabled */
if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
(vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER)
force_thp_readahead = true;
if (!force_thp_readahead) {
/*
* If we don't want any read-ahead, don't bother.
* VM_EXEC case below is already intended for random access.
*/
if ((vm_flags & (VM_RAND_READ | VM_EXEC)) == VM_RAND_READ)
return fpin;
if (!ra->ra_pages)
return fpin;
if (vm_flags & VM_SEQ_READ) {
fpin = maybe_unlock_mmap_for_io(vmf, fpin);
page_cache_sync_ra(&ractl, ra->ra_pages);
return fpin;
}
}
if (!(vm_flags & VM_SEQ_READ)) {
/* Avoid banging the cache line if not needed */
mmap_miss = READ_ONCE(ra->mmap_miss);
if (mmap_miss < MMAP_LOTSAMISS * 10)
WRITE_ONCE(ra->mmap_miss, ++mmap_miss);
/*
* Do we miss much more than hit in this file? If so,
* stop bothering with read-ahead. It will only hurt.
*/
if (mmap_miss > MMAP_LOTSAMISS)
return fpin;
}
if (force_thp_readahead) {
fpin = maybe_unlock_mmap_for_io(vmf, fpin);
ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
ra->size = HPAGE_PMD_NR;
@ -3272,34 +3308,6 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
page_cache_ra_order(&ractl, ra);
return fpin;
}
#endif
/*
* If we don't want any read-ahead, don't bother. VM_EXEC case below is
* already intended for random access.
*/
if ((vm_flags & (VM_RAND_READ | VM_EXEC)) == VM_RAND_READ)
return fpin;
if (!ra->ra_pages)
return fpin;
if (vm_flags & VM_SEQ_READ) {
fpin = maybe_unlock_mmap_for_io(vmf, fpin);
page_cache_sync_ra(&ractl, ra->ra_pages);
return fpin;
}
/* Avoid banging the cache line if not needed */
mmap_miss = READ_ONCE(ra->mmap_miss);
if (mmap_miss < MMAP_LOTSAMISS * 10)
WRITE_ONCE(ra->mmap_miss, ++mmap_miss);
/*
* Do we miss much more than hit in this file? If so,
* stop bothering with read-ahead. It will only hurt.
*/
if (mmap_miss > MMAP_LOTSAMISS)
return fpin;
if (vm_flags & VM_EXEC) {
/*