On Fri, Oct 11, 2019 at 04:41:20PM +0100, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> When doing a buffered write it's possible to leave the subv_writers
> counter of the root, used for synchronization between buffered nocow
> writers and snapshotting. This happens in an exceptional case like the
> following:
>
> 1) We fail to allocate data space for the write, since there's not
> enough available data space nor enough unallocated space for allocating
> a new data block group;
>
> 2) Because of that failure, we try to go to NOCOW mode, which succeeds
> and therefore we set the local variable 'only_release_metadata' to true
> and set the root's sub_writers counter to 1 through the call to
> btrfs_start_write_no_snapshotting() made by check_can_nocow();
>
> 3) The call to btrfs_copy_from_user() returns zero, which is very unlikely
> to happen but not impossible;
>
> 4) No pages are copied because btrfs_copy_from_user() returned zero;
>
> 5) We call btrfs_end_write_no_snapshotting() which decrements the root's
> subv_writers counter to 0;
>
> 6) We don't set 'only_release_metadata' back to 'false' because we do
> it only if 'copied', the value returned by btrfs_copy_from_user(), is
> greater than zero;
>
> 7) On the next iteration of the while loop, which processes the same
> page range, we are now able to allocate data space for the write (we
> got enough data space released in the meanwhile);
>
> 8) After this if we fail at btrfs_delalloc_reserve_metadata(), because
> now there isn't enough free metadata space, or in some other place
> further below (prepare_pages(), lock_and_cleanup_extent_if_need(),
> btrfs_dirty_pages()), we break out of the while loop with
> 'only_release_metadata' having a value of 'true';
>
> 9) Because 'only_release_metadata' is 'true' we end up decrementing the
> root's subv_writers counter to -1 (through a call to
> btrfs_end_write_no_snapshotting()), and we also end up not releasing the
> data space previously reserved through btrfs_check_data_free_space().
> As a consequence the mechanism for synchronizing NOCOW buffered writes
> with snapshotting gets broken.
>
> Fix this by always setting 'only_release_metadata' to false at the start
> of each iteration.
>
> Fixes: 8257b2dc3c1a10 ("Btrfs: introduce btrfs_{start, end}_nocow_write() for each subvolume")
> Fixes: 7ee9e4405f264e ("Btrfs: check if we can nocow if we don't have data space")
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Thanks,
Josef
