On Wed, Oct 09, 2019 at 05:44:22PM +0100, fdman...@kernel.org wrote: > From: Filipe Manana <fdman...@suse.com> > > 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, 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 whenever it > currently has a true value, independently of having been able to copy any > data to the pages.
Can we accomplish the same thing by just doing only_release_metadata = false; at the start of the loop? That way we only ever deal with it in its current scope? Thanks, Josef
