On Mon, Mar 15, 2021 at 7:31 PM David Sterba <dste...@suse.cz> wrote:
>
> On Thu, Mar 11, 2021 at 02:31:06PM +0000, fdman...@kernel.org wrote:
> > From: Filipe Manana <fdman...@suse.com>
> >
> > When freeing a tree block we may end up adding its extent back to the
> > free space cache/tree, as long as there are no more references for it,
> > it was created in the current transaction and writeback for it never
> > happened. This is generally fine, however when we have tree mod log
> > operations it can result in inconsistent versions of a btree after
> > unwinding extent buffers with the recorded tree mod log operations.
> >
> > This is because:
> >
> > * We only log operations for nodes (adding and removing key/pointers),
> > for leaves we don't do anything;
> >
> > * This means that we can log a MOD_LOG_KEY_REMOVE_WHILE_FREEING operation
> > for a node that points to a leaf that was deleted;
> >
> > * Before we apply the logged operation to unwind a node, we can have
> > that leaf's extent allocated again, either as a node or as a leaf, and
> > possibly for another btree. This is possible if the leaf was created in
> > the current transaction and writeback for it never started, in which
> > case btrfs_free_tree_block() returns its extent back to the free space
> > cache/tree;
> >
> > * Then, before applying the tree mod log operation, some task allocates
> > the metadata extent just freed before, and uses it either as a leaf or
> > as a node for some btree (can be the same or another one, it does not
> > matter);
> >
> > * After applying the MOD_LOG_KEY_REMOVE_WHILE_FREEING operation we now
> > get the target node with an item pointing to the metadata extent that
> > now has content different from what it had before the leaf was deleted.
> > It might now belong to a different btree and be a node and not a leaf
> > anymore.
> >
> > As a consequence, the results of searches after the unwinding can be
> > unpredictable and produce unexpected results.
> >
> > So make sure we pin extent buffers corresponding to leaves when there
> > are tree mod log users.
> >
> > Signed-off-by: Filipe Manana <fdman...@suse.com>
> > ---
> > fs/btrfs/extent-tree.c | 23 ++++++++++++++++++++++-
> > 1 file changed, 22 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > index 5e228d6ad63f..2482b26b1971 100644
> > --- a/fs/btrfs/extent-tree.c
> > +++ b/fs/btrfs/extent-tree.c
> > @@ -3310,6 +3310,7 @@ void btrfs_free_tree_block(struct btrfs_trans_handle
> > *trans,
> >
> > if (last_ref && btrfs_header_generation(buf) == trans->transid) {
> > struct btrfs_block_group *cache;
> > + bool must_pin = false;
> >
> > if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID) {
> > ret = check_ref_cleanup(trans, buf->start);
> > @@ -3327,7 +3328,27 @@ void btrfs_free_tree_block(struct btrfs_trans_handle
> > *trans,
> > goto out;
> > }
> >
> > - if (btrfs_is_zoned(fs_info)) {
> > + /*
> > + * If this is a leaf and there are tree mod log users, we may
> > + * have recorded mod log operations that point to this leaf.
> > + * So we must make sure no one reuses this leaf's extent
> > before
> > + * mod log operations are applied to a node, otherwise after
> > + * rewinding a node using the mod log operations we get an
> > + * inconsistent btree, as the leaf's extent may now be used as
> > + * a node or leaf for another different btree.
> > + * We are safe from races here because at this point no other
> > + * node or root points to this extent buffer, so if after this
> > + * check a new tree mod log user joins, it will not be able to
> > + * find a node pointing to this leaf and record operations
> > that
> > + * point to this leaf.
> > + */
> > + if (btrfs_header_level(buf) == 0) {
> > + read_lock(&fs_info->tree_mod_log_lock);
> > + must_pin = !list_empty(&fs_info->tree_mod_seq_list);
> > + read_unlock(&fs_info->tree_mod_log_lock);
> > + }
> > +
> > + if (must_pin || btrfs_is_zoned(fs_info)) {
> > btrfs_redirty_list_add(trans->transaction, buf);
> > pin_down_extent(trans, cache, buf->start, buf->len,
> > 1);
> > btrfs_put_block_group(cache);
>
> This has been added in d3575156f662 ("btrfs: zoned: redirty released
> extent buffers") 5.12-rc1, so it is a regression but otherwise it sounds
> like it's not related only to zoned mode. I'm not sure if this is
> relevant for older stable trees because of missing
> btrfs_redirty_list_add, possibly with some tweaks. Please let me know,
> thanks.
It's not related to zoned filesystems at all.
I just happened to reuse that if branch for the zoned case because it
does the same thing I needed to do, and the call to
btrfs_redirty_list_add() does nothing for the non-zoned case, so it's
safe.
I.e., it's the same as adding the following instead:
if (must_pin) {
pin_down_extent(trans, cache, buf->start, buf->len, 1);
btrfs_put_block_group(cache);
goto out;
}
I opted for the shorter version by ORing the two cases.
But yes, for pre 5.12-rc1 the patch will not apply and the above code
would be needed.
Thanks.
