6.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Josef Bacik <josef@xxxxxxxxxxxxxx>
[ Upstream commit 53e24158684b527d013b5b2204ccb34d1f94c248 ]
Our subpage testing started hanging on generic/560 and I bisected it
down to 1cab1375ba6d ("btrfs: reuse cloned extent buffer during
fiemap to avoid re-allocations"). This is subtle because we use
eb->start to figure out where in the folio we're copying to when we're
subpage, as our ->start may refer to an area inside of the folio.
For example, assume a 16K page size machine with a 4K node size, and
assume that we already have a cloned extent buffer when we cloned the
previous search.
copy_extent_buffer_full() will do the following when copying the extent
buffer path->nodes[0] (src) into cloned (dest):
src->start = 8k; // this is the new leaf we're cloning
cloned->start = 4k; // this is left over from the previous clone
src_addr = folio_address(src->folios[0]);
dest_addr = folio_address(dest->folios[0]);
memcpy(dest_addr + get_eb_offset_in_folio(dst, 0),
src_addr + get_eb_offset_in_folio(src, 0), src->len);
Now get_eb_offset_in_folio() is where the problems occur, because for
sub-pagesize blocksize we can have multiple eb's per folio, the code for
this is as follows
size_t get_eb_offset_in_folio(eb, offset) {
return (eb->start + offset & (folio_size(eb->folio[0]) - 1));
}
So in the above example we are copying into offset 4K inside the folio.
However once we update cloned->start to 8K to match the src the math for
get_eb_offset_in_folio() changes, and any subsequent reads (i.e.
btrfs_item_key_to_cpu()) will start reading from the offset 8K instead
of 4K where we copied to, giving us garbage.
Fix this by setting start before we co copy_extent_buffer_full() to make
sure that we're copying into the same offset inside of the folio that we
will read from later.
All other sites of copy_extent_buffer_full() are correct because we
either set ->start beforehand or we simply don't change it in the case
of the tree-log usage.
With this fix we now pass generic/560 on our subpage tests.
Fixes: 1cab1375ba6d ("btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations")
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
fs/btrfs/extent_io.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 2776112dbdf8d..87f487b116577 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2773,13 +2773,19 @@ static int fiemap_next_leaf_item(struct btrfs_inode *inode, struct btrfs_path *p
goto out;
}
- /* See the comment at fiemap_search_slot() about why we clone. */
- copy_extent_buffer_full(clone, path->nodes[0]);
/*
* Important to preserve the start field, for the optimizations when
* checking if extents are shared (see extent_fiemap()).
+ *
+ * We must set ->start before calling copy_extent_buffer_full(). If we
+ * are on sub-pagesize blocksize, we use ->start to determine the offset
+ * into the folio where our eb exists, and if we update ->start after
+ * the fact then any subsequent reads of the eb may read from a
+ * different offset in the folio than where we originally copied into.
*/
clone->start = path->nodes[0]->start;
+ /* See the comment at fiemap_search_slot() about why we clone. */
+ copy_extent_buffer_full(clone, path->nodes[0]);
slot = path->slots[0];
btrfs_release_path(path);
--
2.43.0
