Re: [PATCH] mm: make mmap_miss accounting symmetric for VM_SEQ_READ

[William Kucharski <william.kucharski@linux.dev>]

Looks good to me.

Reviewed-by: William Kucharski <william.kucharski@linux.dev>

> On May 25, 2026, at 08:57, Usama Arif <usama.arif@linux.dev> wrote:
> 
> 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.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
> mm/filemap.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 5aaba0d3e81d..cca20e350c95 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3433,8 +3433,13 @@ 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);
> @@ -3935,10 +3940,15 @@ 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);
> -- 
> 2.52.0
> 
>