The patch titled Subject: mm: avoid unconditional one-tick sleep when swapcache_prepare fails has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-avoid-unconditional-one-tick-sleep-when-swapcache_prepare-fails.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Barry Song v-songbaohua@oppo.com Subject: mm: avoid unconditional one-tick sleep when swapcache_prepare fails Date: Fri, 27 Sep 2024 09:19:36 +1200

Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache") introduced an unconditional one-tick sleep when `swapcache_prepare()` fails, which has led to reports of UI stuttering on latency-sensitive Android devices. To address this, we can use a waitqueue to wake up tasks that fail `swapcache_prepare()` sooner, instead of always sleeping for a full tick. While tasks may occasionally be woken by an unrelated `do_swap_page()`, this method is preferable to two scenarios: rapid re-entry into page faults, which can cause livelocks, and multiple millisecond sleeps, which visibly degrade user experience.

Oven's testing shows that a single waitqueue resolves the UI stuttering issue. If a 'thundering herd' problem becomes apparent later, a waitqueue hash similar to `folio_wait_table[PAGE_WAIT_TABLE_SIZE]` for page bit locks can be introduced.

Link: https://lkml.kernel.org/r/20240926211936.75373-1-21cnbao@gmail.com Fixes: 13ddaf26be32 ("mm/swap: fix race when skipping swapcache") Signed-off-by: Barry Song v-songbaohua@oppo.com Reported-by: Oven Liyang liyangouwen1@oppo.com Tested-by: Oven Liyang liyangouwen1@oppo.com Cc: Kairui Song kasong@tencent.com Cc: "Huang, Ying" ying.huang@intel.com Cc: Yu Zhao yuzhao@google.com Cc: David Hildenbrand david@redhat.com Cc: Chris Li chrisl@kernel.org Cc: Hugh Dickins hughd@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Michal Hocko mhocko@suse.com Cc: Minchan Kim minchan@kernel.org Cc: Yosry Ahmed yosryahmed@google.com Cc: SeongJae Park sj@kernel.org Cc: Kalesh Singh kaleshsingh@google.com Cc: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memory.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/mm/memory.c~mm-avoid-unconditional-one-tick-sleep-when-swapcache_prepare-fails +++ a/mm/memory.c @@ -4192,6 +4192,8 @@ static struct folio *alloc_swap_folio(st } #endif /* CONFIG_TRANSPARENT_HUGEPAGE */

+static DECLARE_WAIT_QUEUE_HEAD(swapcache_wq); + /* * We enter with non-exclusive mmap_lock (to exclude vma changes, * but allow concurrent faults), and pte mapped but not yet locked. @@ -4204,6 +4206,7 @@ vm_fault_t do_swap_page(struct vm_fault { struct vm_area_struct *vma = vmf->vma; struct folio *swapcache, *folio = NULL; + DECLARE_WAITQUEUE(wait, current); struct page *page; struct swap_info_struct *si = NULL; rmap_t rmap_flags = RMAP_NONE; @@ -4302,7 +4305,9 @@ vm_fault_t do_swap_page(struct vm_fault * Relax a bit to prevent rapid * repeated page faults. */ + add_wait_queue(&swapcache_wq, &wait); schedule_timeout_uninterruptible(1); + remove_wait_queue(&swapcache_wq, &wait); goto out_page; } need_clear_cache = true; @@ -4609,8 +4614,10 @@ unlock: pte_unmap_unlock(vmf->pte, vmf->ptl); out: /* Clear the swap cache pin for direct swapin after PTL unlock */ - if (need_clear_cache) + if (need_clear_cache) { swapcache_clear(si, entry, nr_pages); + wake_up(&swapcache_wq); + } if (si) put_swap_device(si); return ret; @@ -4625,8 +4632,10 @@ out_release: folio_unlock(swapcache); folio_put(swapcache); } - if (need_clear_cache) + if (need_clear_cache) { swapcache_clear(si, entry, nr_pages); + wake_up(&swapcache_wq); + } if (si) put_swap_device(si); return ret; _

Patches currently in -mm which might be from v-songbaohua@oppo.com are

mm-avoid-unconditional-one-tick-sleep-when-swapcache_prepare-fails.patch