mm: add page_counter_margin()

Patch series "mm: avoid large folio splits when swap is unavailable", v7.

This is v7 of Barry's original RFC patch, "mm: Avoiding split large folios
if swap has no space":

https://lore.kernel.org/r/20260618221720.71768-1-baohua@kernel.org

Barry's RFC showed the no-swap case with MADV_PAGEOUT on 16KB mTHP: the
large-folio split counter increased by 1024 even though no swapout
progress was possible.  Skipping the split in that case kept the counter
at 0.

This series makes folio_alloc_swap() classify failures according to
whether splitting a large folio might allow swapout to make progress. 
Callers can then avoid destroying the large folio when neither global swap
availability nor the folio's memcg swap hierarchy has capacity for even a
smaller folio.

Patch #1 adds page_counter_margin(), a small helper that computes the
minimum remaining chargeable space across a page_counter hierarchy.

Patch #2 establishes the folio_alloc_swap() return-value contract:

  - -E2BIG: splitting may let smaller folios make progress
  - -ENOSPC: no global swap space is available
  - -ENOMEM: splitting is not expected to help, including when the
    folio's memcg swap hierarchy has no remaining capacity

Patch #3 makes vmscan split a large folio only when folio_alloc_swap()
returns -E2BIG.  Other failures keep the existing activation path and
avoid destroying the large folio when no smaller part can be backed by
swap either.

Patch #4 applies the same contract to shmem_writeout(), which currently
splits a large folio on every folio_alloc_swap() failure.  It now enters
the split fallback only on -E2BIG; other failures redirty and reactivate
the folio as before.

Testing:

With a 1GB anonymous mapping backed by 16KB mTHPs and memory.swap.max=0,
the patch reduced the median latency of 30 process_madvise(MADV_PAGEOUT)
runs from 743.8 ms to 181.7 ms, while the number of large-folio splits per
run dropped from 65536 to 0.  Neither kernel swapped out any pages.

I also ran DaCapo h2 under swap pressure and found no statistically
significant change in wall time or CPU time.  The overall benefit appears
minor and workload-dependent.


This patch (of 4):

mem_cgroup_get_nr_swap_pages() open-codes the remaining capacity across
the memcg swap counter hierarchy.

Add page_counter_margin() to return the minimum usable space from a page
counter to the root, and use it in mem_cgroup_get_nr_swap_pages().  This
is a pure refactoring with no intended behavior change.

Link: https://lore.kernel.org/20260830042920.2280454-1-xueyuan.chen21@gmail.com
Link: https://lore.kernel.org/20260830042920.2280454-2-xueyuan.chen21@gmail.com
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Nanzhe Zhao <zhaonanzhe@xiaomi.com>
Cc: Youngjun Park <youngjun.park@lge.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index d649b6bbbc87..07b7cb12249c 100644
--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h
@@ -68,6 +68,7 @@ static inline unsigned long page_counter_read(struct page_counter *counter)
 	return atomic_long_read(&counter->usage);
 }
 
+long page_counter_margin(struct page_counter *counter);
 void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages);
 void page_counter_charge(struct page_counter *counter, unsigned long nr_pages);
 bool page_counter_try_charge(struct page_counter *counter,