From f0e9b7d6401959816599191d1d9db90b6fd750db Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Sat, 14 May 2016 09:12:53 +0100 Subject: [PATCH] Btrfs: fix race setting block group readonly during device replace When we do a device replace, for each device extent we find from the source device, we set the corresponding block group to readonly mode to prevent writes into it from happening while we are copying the device extent from the source to the target device. However just before we set the block group to readonly mode some concurrent task might have already allocated an extent from it or decided it could perform a nocow write into one of its extents, which can make the device replace process to miss copying an extent since it uses the extent tree's commit root to search for extents and only once it finishes searching for all extents belonging to the block group it does set the left cursor to the logical end address of the block group - this is a problem if the respective ordered extents finish while we are searching for extents using the extent tree's commit root and no transaction commit happens while we are iterating the tree, since it's the delayed references created by the ordered extents (when they complete) that insert the extent items into the extent tree (using the non-commit root of course). Example: CPU 1 CPU 2 btrfs_dev_replace_start() btrfs_scrub_dev() scrub_enumerate_chunks() --> finds device extent belonging to block group X starts buffered write against some inode writepages is run against that inode forcing dellaloc to run btrfs_writepages() extent_writepages() extent_write_cache_pages() __extent_writepage() writepage_delalloc() run_delalloc_range() cow_file_range() btrfs_reserve_extent() --> allocates an extent from block group X (which is not yet in RO mode) btrfs_add_ordered_extent() --> creates ordered extent Y flush_epd_write_bio() --> bio against the extent from block group X is submitted btrfs_inc_block_group_ro(bg X) --> sets block group X to readonly scrub_chunk(bg X) scrub_stripe(device extent from srcdev) --> keeps searching for extent items belonging to the block group using the extent tree's commit root --> it never blocks due to fs_info->scrub_pause_req as no one tries to commit transaction N --> copies all extents found from the source device into the target device --> finishes search loop bio completes ordered extent Y completes and creates delayed data reference which will add an extent item to the extent tree when run (typically at transaction commit time) --> so the task doing the scrub/device replace at CPU 1 misses this and does not copy this extent into the new/target device btrfs_dec_block_group_ro(bg X) --> turns block group X back to RW mode dev_replace->cursor_left is set to the logical end offset of block group X So fix this by waiting for all cow and nocow writes after setting a block group to readonly mode. Signed-off-by: Filipe Manana Reviewed-by: Josef Bacik --- fs/btrfs/ordered-data.c | 6 +++++- fs/btrfs/ordered-data.h | 2 +- fs/btrfs/scrub.c | 40 ++++++++++++++++++++++++++++++++++++++++ 3 files changed, 46 insertions(+), 2 deletions(-) diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c index 559170464d7c..e96634a725c3 100644 --- a/fs/btrfs/ordered-data.c +++ b/fs/btrfs/ordered-data.c @@ -718,12 +718,13 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr, return count; } -void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr, +int btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr, const u64 range_start, const u64 range_len) { struct btrfs_root *root; struct list_head splice; int done; + int total_done = 0; INIT_LIST_HEAD(&splice); @@ -742,6 +743,7 @@ void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr, done = btrfs_wait_ordered_extents(root, nr, range_start, range_len); btrfs_put_fs_root(root); + total_done += done; spin_lock(&fs_info->ordered_root_lock); if (nr != -1) { @@ -752,6 +754,8 @@ void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr, list_splice_tail(&splice, &fs_info->ordered_roots); spin_unlock(&fs_info->ordered_root_lock); mutex_unlock(&fs_info->ordered_operations_mutex); + + return total_done; } /* diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h index 2049c9be85ee..451507776ff5 100644 --- a/fs/btrfs/ordered-data.h +++ b/fs/btrfs/ordered-data.h @@ -199,7 +199,7 @@ int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr, u32 *sum, int len); int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr, const u64 range_start, const u64 range_len); -void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr, +int btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr, const u64 range_start, const u64 range_len); void btrfs_get_logged_extents(struct inode *inode, struct list_head *logged_list, diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c index 46d847f66e4b..1611572d47bd 100644 --- a/fs/btrfs/scrub.c +++ b/fs/btrfs/scrub.c @@ -3582,6 +3582,46 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx, */ scrub_pause_on(fs_info); ret = btrfs_inc_block_group_ro(root, cache); + if (!ret && is_dev_replace) { + /* + * If we are doing a device replace wait for any tasks + * that started dellaloc right before we set the block + * group to RO mode, as they might have just allocated + * an extent from it or decided they could do a nocow + * write. And if any such tasks did that, wait for their + * ordered extents to complete and then commit the + * current transaction, so that we can later see the new + * extent items in the extent tree - the ordered extents + * create delayed data references (for cow writes) when + * they complete, which will be run and insert the + * corresponding extent items into the extent tree when + * we commit the transaction they used when running + * inode.c:btrfs_finish_ordered_io(). We later use + * the commit root of the extent tree to find extents + * to copy from the srcdev into the tgtdev, and we don't + * want to miss any new extents. + */ + btrfs_wait_block_group_reservations(cache); + btrfs_wait_nocow_writers(cache); + ret = btrfs_wait_ordered_roots(fs_info, -1, + cache->key.objectid, + cache->key.offset); + if (ret > 0) { + struct btrfs_trans_handle *trans; + + trans = btrfs_join_transaction(root); + if (IS_ERR(trans)) + ret = PTR_ERR(trans); + else + ret = btrfs_commit_transaction(trans, + root); + if (ret) { + scrub_pause_off(fs_info); + btrfs_put_block_group(cache); + break; + } + } + } scrub_pause_off(fs_info); if (ret == 0) { -- 2.39.5