Patch "mm/shmem: do not wait for lock_page() in shmem_unused_huge_shrink()" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

mm/shmem: do not wait for lock_page() in shmem_unused_huge_shrink()

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-shmem-do-not-wait-for-lock_page-in-shmem_unused_huge_shrink.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Kirill A. Shutemov" <[email protected]>
Date: Thu, 22 Mar 2018 16:17:35 -0700
Subject: mm/shmem: do not wait for lock_page() in shmem_unused_huge_shrink()

From: Kirill A. Shutemov <[email protected]>

commit b3cd54b257ad95d344d121dc563d943ca39b0921 upstream.

shmem_unused_huge_shrink() gets called from reclaim path. Waiting for page lock may lead to deadlock there.

There was a bug report that may be attributed to this:

http://lkml.kernel.org/r/[email protected]

Replace lock_page() with trylock_page() and skip the page if we failed to lock it. We will get to the page on the next scan.

We can test for the PageTransHuge() outside the page lock as we only need protection against splitting the page under us. Holding pin oni the page is enough for this.

---
 mm/shmem.c |   31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

@@ -493,36 +493,45 @@ next:
 		info = list_entry(pos, struct shmem_inode_info, shrinklist);
 		inode = &info->vfs_inode;
 
-		if (nr_to_split && split >= nr_to_split) {
-			iput(inode);
-			continue;
-		}
+		if (nr_to_split && split >= nr_to_split)
+			goto leave;
 
-		page = find_lock_page(inode->i_mapping,
+		page = find_get_page(inode->i_mapping,
 				(inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT);
 		if (!page)
 			goto drop;
 
+		/* No huge page at the end of the file: nothing to split */
 		if (!PageTransHuge(page)) {
-			unlock_page(page);
 			put_page(page);
 			goto drop;
 		}
 
+		/*
+		 * Leave the inode on the list if we failed to lock
+		 * the page at this time.
+		 *
+		 * Waiting for the lock may lead to deadlock in the
+		 * reclaim path.
+		 */
+		if (!trylock_page(page)) {
+			put_page(page);
+			goto leave;
+		}
+
 		ret = split_huge_page(page);
 		unlock_page(page);
 		put_page(page);
 
-		if (ret) {
-			/* split failed: leave it on the list */
-			iput(inode);
-			continue;
-		}
+		/* If split failed leave the inode on the list */
+		if (ret)
+			goto leave;
 
 		split++;
 drop:
 		list_del_init(&info->shrinklist);
 		removed++;
+leave:
 		iput(inode);
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/mm-khugepaged.c-convert-vm_bug_on-to-collapse-fail.patch queue-4.15/mm-thp-do-not-wait-for-lock_page-in-deferred_split_scan.patch queue-4.15/hugetlbfs-check-for-pgoff-value-overflow.patch queue-4.15/mm-shmem-do-not-wait-for-lock_page-in-shmem_unused_huge_shrink.patch