mm: avoid unconditional one-tick sleep when swapcache_prepare fails

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.

[v-songbaohua@oppo.com: wake_up only when swapcache_wq waitqueue is active]
Link: https://lkml.kernel.org/r/20241008130807.40833-1-21cnbao@gmail.com
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>

diff --git a/mm/memory.c b/mm/memory.c
index 3ccee51adfbbd007b24331fe6874265f231a877b..bdf77a3ec47bc26a8560c41a7ab0d485a9f99367 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4187,6 +4187,8 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 }
 #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.
@@ -4199,6 +4201,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 {
        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;
@@ -4297,7 +4300,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
                                         * 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;
@@ -4604,8 +4609,11 @@ 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);
+               if (waitqueue_active(&swapcache_wq))
+                       wake_up(&swapcache_wq);
+       }
        if (si)
                put_swap_device(si);
        return ret;
@@ -4620,8 +4628,11 @@ out_release:
                folio_unlock(swapcache);
                folio_put(swapcache);
        }
-       if (need_clear_cache)
+       if (need_clear_cache) {
                swapcache_clear(si, entry, nr_pages);
+               if (waitqueue_active(&swapcache_wq))
+                       wake_up(&swapcache_wq);
+       }
        if (si)
                put_swap_device(si);
        return ret;