[PATCH] mm: avoid unconditional one-tick sleep when swapcache_prepare fails

[Barry Song]

From: Barry Song <v-songbaohua@xxxxxxxx>

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.

Fixes: 13ddaf26be32 ("mm/swap: fix race when skipping swapcache")
Cc: Kairui Song <kasong@xxxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Chris Li <chrisl@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: SeongJae Park <sj@xxxxxxxxxx>
Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Oven Liyang <liyangouwen1@xxxxxxxx>
Tested-by: Oven Liyang <liyangouwen1@xxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
 mm/memory.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 2366578015ad..6913174f7f41 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4192,6 +4192,8 @@ 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.
@@ -4204,6 +4206,7 @@ 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;
@@ -4302,7 +4305,9 @@ 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;
@@ -4609,8 +4614,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		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 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		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;
-- 
2.34.1