mm: vmscan: avoid anon scanning for GFP_NOIO with low swapcache

We have observed some cases where memory is allocated with GFP_NOIO, so we
cannot reclaim any anon folios unless they are in swapcache.  We can end
up spending more than 150 ms looping in `shrink_folio_list()` scanning
non-swapcache folios without reclaiming a single folio.  This is pure
overhead.

This is particularly true on systems using zRAM, where swapcache is
relatively rare.  So let's check whether anon reclaim is allowed by GFP_IO
and whether there is enough swapcache to make it worthwhile.  If the
swapcache is extremely low, we're essentially searching for a needle in a
haystack, so let's avoid scanning anon in the first place.

On Android this is triggered by dm-verity hash-block reads through
dm-bufio, which legitimately use GFP_NOIO because they run underneath the
IO path:

  verity_verify_io -> verity_hash_for_block -> verity_verify_level
    -> dm_bufio_read_with_ioprio -> new_read -> __bufio_new
      -> alloc_buffer
         gfp: GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN

Such a reclaimer can land on a memcg with a large, unswapped anon LRU and
a tiny file LRU (e.g.  inactive_anon ~335 MB vs inactive_file ~4 MB, with
negligible swapcache).  shrink_lruvec() then keeps feeding that huge anon
list into shrink_folio_list() - ~2400 shrink_folio_list() calls, ~93,000
anon folios scanned - where every folio is kept because it needs IO.  The
150+ ms above is one such single shrink_lruvec() pass (not accumulated
across a reclaim cycle), and it reclaims nothing; the actual progress
comes entirely from the file side.

Aging anon alongside file does have some value for a later __GFP_IO
reclaimer, so it is not strictly pure overhead.  But that aging is only
deferred, not lost: kswapd and other __GFP_IO reclaimers still walk and
age anon.  Spending ~168 ms aging memory that this context cannot reclaim
is not a worthwhile trade-off in a latency-sensitive path.

To stay conservative, this only skips anon when the swapcache is really
tiny - below 1/64 of the anon LRU - i.e.  when essentially no anon on the
list can be reclaimed without IO.  Whenever there is a meaningful amount
of swapcached anon, the normal path is used and anon is scanned and aged
as before.

Note this only addresses the traditional active/inactive LRU.  MGLRU
selects anon vs file scanning in its own path and is not covered here;
fixing the MGLRU case is left as a TODO.

Link: https://lore.kernel.org/20260908062649.1045883-1-zhangbo56@xiaomi.com
Signed-off-by: Bo Zhang <zhangbo56@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>

0 files changed, 0 insertions, 0 deletions