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 <osan...@osandov.com>
---
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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