FAILED: patch "[PATCH] mm: avoid unconditional one-tick sleep when swapcache_prepare" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 01626a18230246efdcea322aa8f067e60ffe5ccd
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024110505-january-napped-4f1b@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 01626a18230246efdcea322aa8f067e60ffe5ccd Mon Sep 17 00:00:00 2001
From: Barry Song <[email protected]>
Date: Fri, 27 Sep 2024 09:19:36 +1200
Subject: [PATCH] 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.

index 3ccee51adfbb..bdf77a3ec47b 100644
@@ -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 @@ 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);
+		if (waitqueue_active(&swapcache_wq))
+			wake_up(&swapcache_wq);
+	}
 	if (si)
 		put_swap_device(si);
 	return ret;

@@ -4620,8 +4628,11 @@ 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);
+		if (waitqueue_active(&swapcache_wq))
+			wake_up(&swapcache_wq);
+	}
 	if (si)
 		put_swap_device(si);
 	return ret;