github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-07-27 17:47:41 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

mm: make mmap_miss accounting symmetric for VM_SEQ_READ

Browse Source 
do_sync_mmap_readahead() skips both the mmap_miss increment and the
MMAP_LOTSAMISS check for VM_SEQ_READ mappings, since sequential access is
non-speculative and should always read ahead.  The two decrement sites in
do_async_mmap_readahead() and filemap_map_pages() do not mirror this skip,
so concurrent faults on a VM_SEQ_READ mapping can still drive
ra->mmap_miss down to zero through the decrement paths even though nothing
in the sync path ever increments it.  The counter itself is per-file
(file->f_ra.mmap_miss), so it can be moved by any VMA mapping the file,
not just the one currently faulting.

Skip the decrement for VM_SEQ_READ in both decrement sites so the counter
only moves for mappings that also participate in the increment side.  No
functional change for VM_SEQ_READ users, since the increment-side gate
already prevents the counter from being consulted on their behalf, but it
stops a VM_SEQ_READ mapping from biasing the counter for other mappings of
the same file.

Link: https://lore.kernel.org/20260525145751.2671248-1-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Closes: https://lore.kernel.org/all/8edc8cd0-f65c-4456-9b3f-362e744c9a96@linux.dev/
Reviewed-by: William Kucharski <william.kucharski@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Usama Arif 2026-05-25 07:57:51 -07:00 committed by Andrew Morton
parent 6d536ed691
commit eb4c458a98
1 changed files with 12 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
mm/filemap.c
View File

@ -3434,8 +3434,13 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
* Don't touch the mmap_miss counter to avoid decreasing it multiple
* times for a single folio and break the balance with mmap_miss
* increase in do_sync_mmap_readahead().
*
* VM_SEQ_READ mappings skip the mmap_miss increment in
* do_sync_mmap_readahead(), so skip the decrement here as well to
* keep the counter symmetric.
*/
if (likely(!folio_test_locked(folio))) {
if (likely(!folio_test_locked(folio)) &&
!(vmf->vma->vm_flags & VM_SEQ_READ)) {
mmap_miss = READ_ONCE(ra->mmap_miss);
if (mmap_miss)
WRITE_ONCE(ra->mmap_miss, --mmap_miss);
@ -3936,10 +3941,15 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
* In such situation, read-ahead is only a waste of IO.
* Don't decrease mmap_miss in this scenario to make sure
* we can stop read-ahead.
*
* VM_SEQ_READ mappings skip the mmap_miss increment in
* do_sync_mmap_readahead(), so skip the decrement here as
* well to keep the counter symmetric.
*/
if ((map_ret & VM_FAULT_NOPAGE) &&
!(vmf->flags & FAULT_FLAG_TRIED) &&
!folio_test_workingset(folio)) {
!folio_test_workingset(folio) &&
!(vma->vm_flags & VM_SEQ_READ)) {
unsigned short mmap_miss;
mmap_miss = READ_ONCE(file->f_ra.mmap_miss);