On Wed, May 16, 2018 at 04:12:21PM +0200, David Sterba wrote:
> 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.
True, we can easily fix the conflicts by removing @first_key, can't
we?
The problem does exist before it.
thanks,
-liubo
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