writeback_iter() returns a locked folio.  The mapped-folio path redirties
the folio without unlocking it, so shrinking can leave the folio locked
indefinitely.

shmem_write_folio() can also return AOP_WRITEPAGE_ACTIVATE with the folio
locked.  Passing that positive control value back to writeback_iter()
triggers its warning, stops asynchronous writeback, and leaves the folio
locked.  This can occur when zswap rejects a folio while memcg zswap
writeback is disabled.

Unlock folios on both paths and consume AOP_WRITEPAGE_ACTIVATE so the
best-effort writeback scan can continue.

Fixes: 776a853a43c9 ("i915: Use writeback_iter()")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
index ef9440166295..e7b18f7f9644 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
@@ -322,10 +322,16 @@ void __shmem_writeback(size_t size, struct address_space *mapping)
 	 * as normal.
 	 */
 	while ((folio = writeback_iter(mapping, &wbc, folio, &error))) {
-		if (folio_mapped(folio))
+		if (folio_mapped(folio)) {
 			folio_redirty_for_writepage(&wbc, folio);
-		else
+			folio_unlock(folio);
+		} else {
 			error = shmem_write_folio(folio);
+			if (error == AOP_WRITEPAGE_ACTIVATE) {
+				folio_unlock(folio);
+				error = 0;
+			}
+		}
 	}
 }
 

base-commit: cbd3dafc2003db679ccd2f6c6a2551db79657049
-- 
2.55.0