From: Josef Bacik josef@toxicpanda.com
[ 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.
CC: stable@vger.kernel.org # 6.1 Fixes: 1cab1375ba6d ("btrfs: reuse cloned extent buffer during fiemap to avoid re-allocations") Reviewed-by: Filipe Manana fdmanana@suse.com Reviewed-by: Qu Wenruo wqu@suse.com Signed-off-by: Josef Bacik josef@toxicpanda.com Signed-off-by: David Sterba dsterba@suse.com --- 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 c07d12184ae2..8172cc527760 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -3831,13 +3831,19 @@ 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);