[PATCH] squashfs: fix cache race with migration

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

Migration replaces the page in the mapping before copying the contents
and the flags over from the old page, so check that the page in the page
cache is really up to date before using it.  Without this, stressing
squashfs reads with parallel compaction sometimes results in squashfs
reporting data corruption.

Fixes: e994f5b677ee ("squashfs: cache partial compressed blocks")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
 fs/squashfs/block.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/fs/squashfs/block.c b/fs/squashfs/block.c
index 6aa9c2e1e8ebe..581ce95193390 100644
--- a/fs/squashfs/block.c
+++ b/fs/squashfs/block.c
@@ -166,6 +166,26 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
 	return 0;
 }
 
+static struct page *squashfs_get_cache_page(struct address_space *mapping,
+					    pgoff_t index)
+{
+	struct page *page;
+
+	if (!mapping)
+		return NULL;
+
+	page = find_get_page(mapping, index);
+	if (!page)
+		return NULL;
+
+	if (!PageUptodate(page)) {
+		put_page(page);
+		return NULL;
+	}
+
+	return page;
+}
+
 static int squashfs_bio_read(struct super_block *sb, u64 index, int length,
 			     struct bio **biop, int *block_offset)
 {
@@ -190,11 +210,10 @@ static int squashfs_bio_read(struct super_block *sb, u64 index, int length,
 	for (i = 0; i < page_count; ++i) {
 		unsigned int len =
 			min_t(unsigned int, PAGE_SIZE - offset, total_len);
-		struct page *page = NULL;
+		pgoff_t index = (read_start >> PAGE_SHIFT) + i;
+		struct page *page;
 
-		if (cache_mapping)
-			page = find_get_page(cache_mapping,
-					     (read_start >> PAGE_SHIFT) + i);
+		page = squashfs_get_cache_page(cache_mapping, index);
 		if (!page)
 			page = alloc_page(GFP_NOIO);
 

---
base-commit: 3a8a670eeeaa40d87bd38a587438952741980c18
change-id: 20230629-squashfs-cache-migration-27154a50f449

Best regards,
-- 
Vincent Whitchurch <vincent.whitchurch@axis.com>