| Message ID | c655306f61af9b2d75ed22053a7cdc3f21022d72.1611337435.git.fdmanana@suse.com (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Headers | show |
| Series | btrfs: fix log replay failure due to race with space cache rebuild | expand |
On 1/22/21 12:56 PM, fdmanana@kernel.org wrote: > From: Filipe Manana <fdmanana@suse.com> > > After a sudden power failure we may end up with a space cache on disk that > is not valid and needs to be rebuilt from scratch. > > If that happens, during log replay when we attempt to pin an extent buffer > from a log tree, at btrfs_pin_extent_for_log_replay(), we do not wait for > the space cache to be rebuilt through the call to: > > btrfs_cache_block_group(cache, 1); > > That is because that only waits for the task (work queue job) that loads > the space cache to change the cache state from BTRFS_CACHE_FAST to any > other value. That is ok when the space cache on disk exists and is valid, > but when the cache is not valid and needs to be rebuilt, it ends up > returning as soon as the cache state changes to BTRFS_CACHE_STARTED (done > at caching_thread()). > > So this means that we can end up trying to unpin a range which is not yet > marked as free in the block group. This results in the call to > btrfs_remove_free_space() to return -EINVAL to > btrfs_pin_extent_for_log_replay(), which in turn makes the log replay fail > as well as mounting the filesystem. More specifically the -EINVAL comes > from free_space_cache.c:remove_from_bitmap(), because the requested range > is not marked as free space (ones in the bitmap), we have the following > condition triggered: > > static noinline int remove_from_bitmap(struct btrfs_free_space_ctl *ctl, > (...) > if (ret < 0 || search_start != *offset) > return -EINVAL; > (...) > > It's the "search_start != *offset" that results in the condition being > evaluated to true. > > When this happens we got the following in dmesg/syslog: > > [72383.415114] BTRFS: device fsid 32b95b69-0ea9-496a-9f02-3f5a56dc9322 devid 1 transid 1432 /dev/sdb scanned by mount (3816007) > [72383.417837] BTRFS info (device sdb): disk space caching is enabled > [72383.418536] BTRFS info (device sdb): has skinny extents > [72383.423846] BTRFS info (device sdb): start tree-log replay > [72383.426416] BTRFS warning (device sdb): block group 30408704 has wrong amount of free space > [72383.427686] BTRFS warning (device sdb): failed to load free space cache for block group 30408704, rebuilding it now > [72383.454291] BTRFS: error (device sdb) in btrfs_recover_log_trees:6203: errno=-22 unknown (Failed to pin buffers while recovering log root tree.) > [72383.456725] BTRFS: error (device sdb) in btrfs_replay_log:2253: errno=-22 unknown (Failed to recover log tree) > [72383.460241] BTRFS error (device sdb): open_ctree failed > > We also mark the range for the extent buffer in the excluded extents io > tree. That is fine when the space cache is valid on disk and we can load > it, in which case it causes no problems. > > However, for the case where we need to rebuild the space cache, because it > is either invalid or it is missing, having the extent buffer range marked > in the excluded extents io tree leads to a -EINVAL failure from the call > to btrfs_remove_free_space(), resulting in the log replay and mount to > fail. This is because by having the range marked in the excluded extents > io tree, the caching thread ends up never adding the range of the extent > buffer as free space in the block group since the calls to > add_new_free_space(), called from load_extent_tree_free(), filter out any > ranges that are marked as excluded extents. > > So fix this by making sure that during log replay we wait for the caching > task to finish completely when we need to rebuild a space cache, and also > drop the need to mark the extent buffer range in the excluded extents io > tree, as well as clearing ranges from that tree at > btrfs_finish_extent_commit(). > > This started to happen with some frequency on large filesystems having > block groups with a lot of fragmentation since the recent commit > e747853cae3ae3 ("btrfs: load free space cache asynchronously"), but in > fact the issue has been there for years, it was just much less likely > to happen. > > Signed-off-by: Filipe Manana <fdmanana@suse.com> > --- > fs/btrfs/extent-tree.c | 27 ++++++++++++++++++--------- > 1 file changed, 18 insertions(+), 9 deletions(-) > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > index 30b1a630dc2f..89d1b0551cf8 100644 > --- a/fs/btrfs/extent-tree.c > +++ b/fs/btrfs/extent-tree.c > @@ -2602,8 +2602,6 @@ int btrfs_pin_extent_for_log_replay(struct btrfs_trans_handle *trans, > struct btrfs_block_group *cache; > int ret; > > - btrfs_add_excluded_extent(trans->fs_info, bytenr, num_bytes); > - > cache = btrfs_lookup_block_group(trans->fs_info, bytenr); > if (!cache) > return -EINVAL; > @@ -2615,6 +2613,15 @@ int btrfs_pin_extent_for_log_replay(struct btrfs_trans_handle *trans, > * the pinned extents. > */ > btrfs_cache_block_group(cache, 1); > + /* > + * Make sure we wait until the cache is completely built in case it is > + * missing or is invalid and therefore needs to be rebuilt. > + */ > + if (btrfs_test_opt(trans->fs_info, SPACE_CACHE)) { > + ret = btrfs_wait_block_group_cache_done(cache); > + if (ret) > + return ret; > + } Sorry I didn't explain this well in my previous reply and on slack. If we are going to keep the btrfs_remove_free_space() below, which we have to, we cannot remove the btrfs_add_excluded_extent() without also unconditionally loading the space cache. The 'load_cache_only' only means we'll wait for the space cache to be loaded in the SPACE_CACHE case, not that we won't start caching. Consider the free space tree case, we'll be doing the normal caching, and we'll either hit the case that you're trying to fix, because we attempt to remove a free space range that has been partially cached in the bitmap. Or we'll hit the case where we think we've removed the free space even though it's not been loaded yet. Your fix needs to remove the excluded extent parts as well as do btrfs_cache_block_group(cache, 0); btrfs_wait_block_group_cache_done(cache); in order to be properly safe in all cases. Thanks, Josef
On Fri, Jan 22, 2021 at 6:39 PM Josef Bacik <josef@toxicpanda.com> wrote: > > On 1/22/21 12:56 PM, fdmanana@kernel.org wrote: > > From: Filipe Manana <fdmanana@suse.com> > > > > After a sudden power failure we may end up with a space cache on disk that > > is not valid and needs to be rebuilt from scratch. > > > > If that happens, during log replay when we attempt to pin an extent buffer > > from a log tree, at btrfs_pin_extent_for_log_replay(), we do not wait for > > the space cache to be rebuilt through the call to: > > > > btrfs_cache_block_group(cache, 1); > > > > That is because that only waits for the task (work queue job) that loads > > the space cache to change the cache state from BTRFS_CACHE_FAST to any > > other value. That is ok when the space cache on disk exists and is valid, > > but when the cache is not valid and needs to be rebuilt, it ends up > > returning as soon as the cache state changes to BTRFS_CACHE_STARTED (done > > at caching_thread()). > > > > So this means that we can end up trying to unpin a range which is not yet > > marked as free in the block group. This results in the call to > > btrfs_remove_free_space() to return -EINVAL to > > btrfs_pin_extent_for_log_replay(), which in turn makes the log replay fail > > as well as mounting the filesystem. More specifically the -EINVAL comes > > from free_space_cache.c:remove_from_bitmap(), because the requested range > > is not marked as free space (ones in the bitmap), we have the following > > condition triggered: > > > > static noinline int remove_from_bitmap(struct btrfs_free_space_ctl *ctl, > > (...) > > if (ret < 0 || search_start != *offset) > > return -EINVAL; > > (...) > > > > It's the "search_start != *offset" that results in the condition being > > evaluated to true. > > > > When this happens we got the following in dmesg/syslog: > > > > [72383.415114] BTRFS: device fsid 32b95b69-0ea9-496a-9f02-3f5a56dc9322 devid 1 transid 1432 /dev/sdb scanned by mount (3816007) > > [72383.417837] BTRFS info (device sdb): disk space caching is enabled > > [72383.418536] BTRFS info (device sdb): has skinny extents > > [72383.423846] BTRFS info (device sdb): start tree-log replay > > [72383.426416] BTRFS warning (device sdb): block group 30408704 has wrong amount of free space > > [72383.427686] BTRFS warning (device sdb): failed to load free space cache for block group 30408704, rebuilding it now > > [72383.454291] BTRFS: error (device sdb) in btrfs_recover_log_trees:6203: errno=-22 unknown (Failed to pin buffers while recovering log root tree.) > > [72383.456725] BTRFS: error (device sdb) in btrfs_replay_log:2253: errno=-22 unknown (Failed to recover log tree) > > [72383.460241] BTRFS error (device sdb): open_ctree failed > > > > We also mark the range for the extent buffer in the excluded extents io > > tree. That is fine when the space cache is valid on disk and we can load > > it, in which case it causes no problems. > > > > However, for the case where we need to rebuild the space cache, because it > > is either invalid or it is missing, having the extent buffer range marked > > in the excluded extents io tree leads to a -EINVAL failure from the call > > to btrfs_remove_free_space(), resulting in the log replay and mount to > > fail. This is because by having the range marked in the excluded extents > > io tree, the caching thread ends up never adding the range of the extent > > buffer as free space in the block group since the calls to > > add_new_free_space(), called from load_extent_tree_free(), filter out any > > ranges that are marked as excluded extents. > > > > So fix this by making sure that during log replay we wait for the caching > > task to finish completely when we need to rebuild a space cache, and also > > drop the need to mark the extent buffer range in the excluded extents io > > tree, as well as clearing ranges from that tree at > > btrfs_finish_extent_commit(). > > > > This started to happen with some frequency on large filesystems having > > block groups with a lot of fragmentation since the recent commit > > e747853cae3ae3 ("btrfs: load free space cache asynchronously"), but in > > fact the issue has been there for years, it was just much less likely > > to happen. > > > > Signed-off-by: Filipe Manana <fdmanana@suse.com> > > --- > > fs/btrfs/extent-tree.c | 27 ++++++++++++++++++--------- > > 1 file changed, 18 insertions(+), 9 deletions(-) > > > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > > index 30b1a630dc2f..89d1b0551cf8 100644 > > --- a/fs/btrfs/extent-tree.c > > +++ b/fs/btrfs/extent-tree.c > > @@ -2602,8 +2602,6 @@ int btrfs_pin_extent_for_log_replay(struct btrfs_trans_handle *trans, > > struct btrfs_block_group *cache; > > int ret; > > > > - btrfs_add_excluded_extent(trans->fs_info, bytenr, num_bytes); > > - > > cache = btrfs_lookup_block_group(trans->fs_info, bytenr); > > if (!cache) > > return -EINVAL; > > @@ -2615,6 +2613,15 @@ int btrfs_pin_extent_for_log_replay(struct btrfs_trans_handle *trans, > > * the pinned extents. > > */ > > btrfs_cache_block_group(cache, 1); > > + /* > > + * Make sure we wait until the cache is completely built in case it is > > + * missing or is invalid and therefore needs to be rebuilt. > > + */ > > + if (btrfs_test_opt(trans->fs_info, SPACE_CACHE)) { > > + ret = btrfs_wait_block_group_cache_done(cache); > > + if (ret) > > + return ret; > > + } > > Sorry I didn't explain this well in my previous reply and on slack. > > If we are going to keep the btrfs_remove_free_space() below, which we have to, > we cannot remove the btrfs_add_excluded_extent() without also unconditionally > loading the space cache. The 'load_cache_only' only means we'll wait for the > space cache to be loaded in the SPACE_CACHE case, not that we won't start caching. > > Consider the free space tree case, we'll be doing the normal caching, and we'll > either hit the case that you're trying to fix, because we attempt to remove a > free space range that has been partially cached in the bitmap. Or we'll hit the > case where we think we've removed the free space even though it's not been > loaded yet. > > Your fix needs to remove the excluded extent parts as well as do > > btrfs_cache_block_group(cache, 0); > btrfs_wait_block_group_cache_done(cache); > > in order to be properly safe in all cases. Thanks, Yes, I realized that later, I have a new version being tested right now: https://pastebin.com/pgQ2y11a I messed and didn't realize the wait is necessary even if there is no space cache or tree enabled. > > Josef
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 30b1a630dc2f..89d1b0551cf8 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -2602,8 +2602,6 @@ int btrfs_pin_extent_for_log_replay(struct btrfs_trans_handle *trans, struct btrfs_block_group *cache; int ret; - btrfs_add_excluded_extent(trans->fs_info, bytenr, num_bytes); - cache = btrfs_lookup_block_group(trans->fs_info, bytenr); if (!cache) return -EINVAL; @@ -2615,6 +2613,15 @@ int btrfs_pin_extent_for_log_replay(struct btrfs_trans_handle *trans, * the pinned extents. */ btrfs_cache_block_group(cache, 1); + /* + * Make sure we wait until the cache is completely built in case it is + * missing or is invalid and therefore needs to be rebuilt. + */ + if (btrfs_test_opt(trans->fs_info, SPACE_CACHE)) { + ret = btrfs_wait_block_group_cache_done(cache); + if (ret) + return ret; + } pin_down_extent(trans, cache, bytenr, num_bytes, 0); @@ -2635,7 +2642,7 @@ static int __exclude_logged_extent(struct btrfs_fs_info *fs_info, if (!block_group) return -EINVAL; - btrfs_cache_block_group(block_group, 0); + btrfs_cache_block_group(block_group, 1); caching_ctl = btrfs_get_caching_control(block_group); if (!caching_ctl) { @@ -2644,10 +2651,15 @@ static int __exclude_logged_extent(struct btrfs_fs_info *fs_info, ret = btrfs_remove_free_space(block_group, start, num_bytes); } else { /* - * We must wait for v1 caching to finish, otherwise we may not - * remove our space. + * Make sure we wait until the cache is completely built in case it is + * missing or is invalid and therefore needs to be rebuilt. */ - btrfs_wait_space_cache_v1_finished(block_group, caching_ctl); + if (btrfs_test_opt(fs_info, SPACE_CACHE)) { + ret = btrfs_wait_block_group_cache_done(block_group); + if (ret) + return ret; + } + mutex_lock(&caching_ctl->mutex); if (start >= caching_ctl->progress) { @@ -2863,9 +2875,6 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans) mutex_unlock(&fs_info->unused_bg_unpin_mutex); break; } - if (test_bit(BTRFS_FS_LOG_RECOVERING, &fs_info->flags)) - clear_extent_bits(&fs_info->excluded_extents, start, - end, EXTENT_UPTODATE); if (btrfs_test_opt(fs_info, DISCARD_SYNC)) ret = btrfs_discard_extent(fs_info, start,