FAILED: patch "[PATCH] afs: Fix potential thrashing in afs writeback" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-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]>.

thanks,

greg k-h

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

From 173ce1ca47c489135b2799f70f550e1319ba36d8 Mon Sep 17 00:00:00 2001
From: David Howells <[email protected]>
Date: Fri, 11 Mar 2022 15:58:21 +0000
Subject: [PATCH] afs: Fix potential thrashing in afs writeback

In afs_writepages_region(), if the dirty page we find is undergoing writeback or write to cache, but the sync_mode is WB_SYNC_NONE, we go round the loop trying the same page again and again with no pausing or waiting unless and until another thread manages to clear the writeback and fscache flags.

Fix this with three measures:

(1) Advance start to after the page we found.

(2) Break out of the loop and return if rescheduling is requested.

(3) Arbitrarily give up after a maximum of 5 skips.

index 5e9157d0da29..f447c902318d 100644
@@ -703,7 +703,7 @@ static int afs_writepages_region(struct address_space *mapping,
 	struct folio *folio;
 	struct page *head_page;
 	ssize_t ret;
-	int n;
+	int n, skips = 0;
 
 	_enter("%llx,%llx,", start, end);
 

@@ -754,8 +754,15 @@ static int afs_writepages_region(struct address_space *mapping,
 #ifdef CONFIG_AFS_FSCACHE
 				folio_wait_fscache(folio);
 #endif
+			} else {
+				start += folio_size(folio);
 			}
 			folio_put(folio);
+			if (wbc->sync_mode == WB_SYNC_NONE) {
+				if (skips >= 5 || need_resched())
+					break;
+				skips++;
+			}
 			continue;
 		}