[PATCH 2/3] btrfs: handle race on ENOMEM in alloc_extent_buffer

[Omar Sandoval]

Consider the following interleaving of overlapping calls to
alloc_extent_buffer:

Call 1:

- Successfully allocates a few pages with find_or_create_page
- find_or_create_page fails, goto free_eb
- Unlocks the allocated pages

Call 2:
- Calls find_or_create_page and gets a page in call 1's extent_buffer
- Finds that the page is already associated with an extent_buffer
- Grabs a reference to the half-written extent_buffer and calls
  mark_extent_buffer_accessed on it

mark_extent_buffer_accessed will then try to call mark_page_accessed on
a null page and panic.

The fix is to clear page->private of the half-written extent_buffer's
pages all at once while holding mapping->private_lock.

Signed-off-by: Omar Sandoval <osandov@xxxxxxxxxxx>
---
 fs/btrfs/extent_io.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index c73df6a..6024db9 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4850,6 +4850,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 				mark_extent_buffer_accessed(exists, p);
 				goto free_eb;
 			}
+			exists = NULL;
 
 			/*
 			 * Do this so attach doesn't complain and we need to
@@ -4913,13 +4914,24 @@ again:
 	return eb;
 
 free_eb:
+	spin_lock(&mapping->private_lock);
 	for (i = 0; i < num_pages; i++) {
-		if (eb->pages[i])
-			unlock_page(eb->pages[i]);
-	}
+		struct page *page = eb->pages[i];
 
+		if (page) {
+			unlock_page(page);
+			ClearPagePrivate(page);
+			set_page_private(page, 0);
+			/* One for the page private */
+			page_cache_release(page);
+			/* One for when we alloced the page */
+			page_cache_release(page);
+		}
+	}
+	spin_unlock(&mapping->private_lock);
 	WARN_ON(!atomic_dec_and_test(&eb->refs));
-	btrfs_release_extent_buffer(eb);
+	__free_extent_buffer(eb);
+
 	return exists;
 }
 
-- 
2.3.0

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