From: Josef Bacik <josef@toxicpanda.com>
To: linux-btrfs@vger.kernel.org, kernel-team@fb.com
Subject: [PATCH v2] btrfs: set start on clone before calling copy_extent_buffer_full
Date: Sun, 14 Apr 2024 09:22:38 -0400 [thread overview]
Message-ID: <cbf5bf79edc537544f383ee3d6c79a1bec45a964.1713100883.git.josef@toxicpanda.com> (raw)
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 (ie
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")
Signed-off-by: Josef Bacik <josef@toxicpanda.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 49f7161a6578..a59cd88cf318 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2809,13 +2809,19 @@ 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
next reply other threads:[~2024-04-14 13:22 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-14 13:22 Josef Bacik [this message]
2024-04-15 0:04 ` [PATCH v2] btrfs: set start on clone before calling copy_extent_buffer_full Qu Wenruo
2024-04-15 11:33 ` Filipe Manana
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