On Wed, May 16, 2018 at 11:00:14AM +0300, Nikolay Borisov wrote:
>
>
> On 15.05.2018 20:37, Liu Bo wrote:
> > If a btree block, aka. extent buffer, is not available in the extent
> > buffer cache, it'll be read out from the disk instead, i.e.
> >
> > btrfs_search_slot()
> > read_block_for_search() # hold parent and its lock, go to read child
> > btrfs_release_path()
> > read_tree_block() # read child
> >
> > Unfortunately, the parent lock got released before reading child, so
> > commit 5bdd3536cbbe ("Btrfs: Fix block generation verification race") had
> > used 0 as parent transid to read the child block. It forces
> > read_tree_block() not to check if parent transid is different with the
> > generation id of the child that it reads out from disk.
> >
> > A simple PoC is included in btrfs/124,
> >
> > 0. A two-disk raid1 btrfs,
> >
> > 1. Right after mkfs.btrfs, block A is allocated to be device tree's root.
> >
> > 2. Mount this filesystem and put it in use, after a while, device tree's
> > root got COW but block A hasn't been allocated/overwritten yet.
> >
> > 3. Umount it and reload the btrfs module to remove both disks from the
> > global @fs_devices list.
> >
> > 4. mount -odegraded dev1 and write some data, so now block A is allocated
> > to be a leaf in checksum tree. Note that only dev1 has the latest
> > metadata of this filesystem.
> >
> > 5. Umount it and mount it again normally (with both disks), since raid1
> > can pick up one disk by the writer task's pid, if btrfs_search_slot()
> > needs to read block A, dev2 which does NOT have the latest metadata
> > might be read for block A, then we got a stale block A.
> >
> > 6. As parent transid is not checked, block A is marked as uptodate and
> > put into the extent buffer cache, so the future search won't bother
> > to read disk again, which means it'll make changes on this stale
> > one and make it dirty and flush it onto disk.
> >
> > To avoid the problem, parent transid needs to be passed to
> > read_tree_block().
> >
> > In order to get a valid parent transid, we need to hold the parent's
> > lock until finish reading child.
> >
> > Signed-off-by: Liu Bo <bo.liu@xxxxxxxxxxxxxxxxx>
>
> Doesn't this warrant a stable tag and
> Fixes: 5bdd3536cbbe ("Btrfs: Fix block generation verification race")
The patch will not apply to < 4.16 as it depends on the addition of
&first_key check from 581c1760415c4, but yes we need it for stable.
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