FAILED: patch "[PATCH] mm/shmem: fix accounting of swapin-error markers" failed to apply to 6.1-stable tree

FAILED: patch "[PATCH] mm/shmem: fix accounting of swapin-error markers" failed to apply to 6.1-stable tree

[Sasha Levin]

The patch below does not apply to the 6.1-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 <stable@vger.kernel.org>.

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-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 66ce68a249f4ac56cd6968aa6d8ded20fb4b7d09
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109201550-failed-6.1-66ce68a249f4@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

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

From 66ce68a249f4ac56cd6968aa6d8ded20fb4b7d09 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Thu, 29 Oct 2026 11:43:35 +0100
Subject: [PATCH] mm/shmem: fix accounting of swapin-error markers

A failed swap read replaces the swap entry with a swapin-error marker in
the page-cache and subtracts its pages from "alloced" and "swapped";
however, it leaves their block charge in place.

Removing the marker during truncate then subtracts from "swapped" again.
This can underflow the counters or make swapped reach zero while other
pages remain swapped out, causing swapoff to loop without finding them.

When installing the marker, subtract only from "swapped" and let
shmem_recalc_inode() reduce "alloced" and release the block charge.

Make shmem_free_swap() report successful removal separately from the
number of swap pages freed, so removing a swapin-error marker does not
subtract from "swapped" again.

It's a bit confusing, but replacing a swapped out page by a swapin-error
marker in shmem_set_folio_swapin_error() behaves as follows:

Initially:
	alloced=1, swapped=1, nrpages=0
Calling shmem_recalc_inode(inode, 0, -1)
	-> swapped = 0
	-> freed =  1 - 0 - 0 = 1
	-> alloced -= freed
	-> alloced=0, swapped=0, block charge released

Fixes: 6cec2b95dadf ("mm/shmem: fix infinite loop when swap in shmem error at swapoff time")
Cc: stable@vger.kernel.org
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Link: https://patch.msgid.link/20261029-fixes_shmem-v4-14-392fc715d8f6@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index eb2e6e50630c0..897f19ed0fe3d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1177,12 +1177,11 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
 }
 
 /*
- * Remove swap entry from page cache, free the swap and its page cache. Returns
- * the number of pages being freed. 0 means entry not found in XArray (0 pages
- * being freed).
+ * Remove a swap entry or swapin-error marker wholly within [index, end].
+ * Return whether it was removed, counting freed swap pages in nr_swaps_freed.
  */
-static long shmem_free_swap(struct address_space *mapping,
-			    pgoff_t index, pgoff_t end, void *radswap)
+static bool shmem_free_swap(struct address_space *mapping,
+		pgoff_t index, pgoff_t end, void *radswap, long *nr_swaps_freed)
 {
 	XA_STATE(xas, &mapping->i_pages, index);
 	const softleaf_t swp = radix_to_swp_entry(radswap);
@@ -1203,8 +1202,10 @@ static long shmem_free_swap(struct address_space *mapping,
 	xas_unlock_irq(&xas);
 
 	/* A swapin-error marker holds no swap slot, so just drop it. */
-	if (nr_pages && softleaf_is_swap(swp))
+	if (nr_pages && softleaf_is_swap(swp)) {
 		swap_put_entries_direct(swp, nr_pages);
+		*nr_swaps_freed += nr_pages;
+	}
 
 	return nr_pages;
 }
@@ -1358,8 +1359,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 			if (xa_is_value(folio)) {
 				if (unfalloc)
 					continue;
-				nr_swaps_freed += shmem_free_swap(mapping, indices[i],
-								  end - 1, folio);
+				shmem_free_swap(mapping, indices[i], end - 1,
+						folio, &nr_swaps_freed);
 				continue;
 			}
 
@@ -1422,13 +1423,11 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 
 			if (xa_is_value(folio)) {
 				int order;
-				long swaps_freed;
 
 				if (unfalloc)
 					continue;
-				swaps_freed = shmem_free_swap(mapping, indices[i],
-							      end - 1, folio);
-				if (!swaps_freed) {
+				if (!shmem_free_swap(mapping, indices[i], end - 1,
+						     folio, &nr_swaps_freed)) {
 					pgoff_t base = indices[i];
 
 					order = shmem_confirm_swap(mapping, indices[i],
@@ -1449,7 +1448,6 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 					index = base;
 					break;
 				}
-				nr_swaps_freed += swaps_freed;
 				continue;
 			}
 
@@ -2409,11 +2407,10 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
 	folio_put_swap(folio, NULL);
 	swap_cache_del_folio(folio);
 	/*
-	 * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
-	 * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
-	 * in shmem_evict_inode().
+	 * Let shmem_recalc_inode() reduce alloced and release the block charge
+	 * for the freed swap pages.
 	 */
-	shmem_recalc_inode(inode, -nr_pages, -nr_pages);
+	shmem_recalc_inode(inode, 0, -nr_pages);
 }
 
 static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
-- 
2.53.0