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authorQu Wenruo <wqu@suse.com>2020-01-24 07:58:20 +0800
committerDavid Sterba <dsterba@suse.com>2020-01-24 14:35:56 +0100
commit1bbb97b8ce7ddf3a56645636c905cef706706dd9 (patch)
treede78fde5a80b76366fc09da518a9ee9215689b6a
parentb35cf1f0bf1f2b0b193093338414b9bd63b29015 (diff)
downloadlinux-1bbb97b8ce7ddf3a56645636c905cef706706dd9.tar.gz
btrfs: scrub: Require mandatory block group RO for dev-replace
[BUG] For dev-replace test cases with fsstress, like btrfs/06[45] btrfs/071, looped runs can lead to random failure, where scrub finds csum error. The possibility is not high, around 1/20 to 1/100, but it's causing data corruption. The bug is observable after commit b12de52896c0 ("btrfs: scrub: Don't check free space before marking a block group RO") [CAUSE] Dev-replace has two source of writes: - Write duplication All writes to source device will also be duplicated to target device. Content: Not yet persisted data/meta - Scrub copy Dev-replace reused scrub code to iterate through existing extents, and copy the verified data to target device. Content: Previously persisted data and metadata The difference in contents makes the following race possible: Regular Writer | Dev-replace ----------------------------------------------------------------- ^ | | Preallocate one data extent | | at bytenr X, len 1M | v | ^ Commit transaction | | Now extent [X, X+1M) is in | v commit root | ================== Dev replace starts ========================= | ^ | | Scrub extent [X, X+1M) | | Read [X, X+1M) | | (The content are mostly garbage | | since it's preallocated) ^ | v | Write back happens for | | extent [X, X+512K) | | New data writes to both | | source and target dev. | v | | ^ | | Scrub writes back extent [X, X+1M) | | to target device. | | This will over write the new data in | | [X, X+512K) | v This race can only happen for nocow writes. Thus metadata and data cow writes are safe, as COW will never overwrite extents of previous transaction (in commit root). This behavior can be confirmed by disabling all fallocate related calls in fsstress (*), then all related tests can pass a 2000 run loop. *: FSSTRESS_AVOID="-f fallocate=0 -f allocsp=0 -f zero=0 -f insert=0 \ -f collapse=0 -f punch=0 -f resvsp=0" I didn't expect resvsp ioctl will fallback to fallocate in VFS... [FIX] Make dev-replace to require mandatory block group RO, and wait for current nocow writes before calling scrub_chunk(). This patch will mostly revert commit 76a8efa171bf ("btrfs: Continue replace when set_block_ro failed") for dev-replace path. The side effect is, dev-replace can be more strict on avaialble space, but definitely worth to avoid data corruption. Reported-by: Filipe Manana <fdmanana@suse.com> Fixes: 76a8efa171bf ("btrfs: Continue replace when set_block_ro failed") Fixes: b12de52896c0 ("btrfs: scrub: Don't check free space before marking a block group RO") Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Qu Wenruo <wqu@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
-rw-r--r--fs/btrfs/scrub.c33
1 files changed, 28 insertions, 5 deletions
diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 21de630b0730..fd266a2d15ec 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3577,17 +3577,27 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
* This can easily boost the amount of SYSTEM chunks if cleaner
* thread can't be triggered fast enough, and use up all space
* of btrfs_super_block::sys_chunk_array
+ *
+ * While for dev replace, we need to try our best to mark block
+ * group RO, to prevent race between:
+ * - Write duplication
+ * Contains latest data
+ * - Scrub copy
+ * Contains data from commit tree
+ *
+ * If target block group is not marked RO, nocow writes can
+ * be overwritten by scrub copy, causing data corruption.
+ * So for dev-replace, it's not allowed to continue if a block
+ * group is not RO.
*/
- ret = btrfs_inc_block_group_ro(cache, false);
- scrub_pause_off(fs_info);
-
+ ret = btrfs_inc_block_group_ro(cache, sctx->is_dev_replace);
if (ret == 0) {
ro_set = 1;
- } else if (ret == -ENOSPC) {
+ } else if (ret == -ENOSPC && !sctx->is_dev_replace) {
/*
* btrfs_inc_block_group_ro return -ENOSPC when it
* failed in creating new chunk for metadata.
- * It is not a problem for scrub/replace, because
+ * It is not a problem for scrub, because
* metadata are always cowed, and our scrub paused
* commit_transactions.
*/
@@ -3596,9 +3606,22 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
btrfs_warn(fs_info,
"failed setting block group ro: %d", ret);
btrfs_put_block_group(cache);
+ scrub_pause_off(fs_info);
break;
}
+ /*
+ * Now the target block is marked RO, wait for nocow writes to
+ * finish before dev-replace.
+ * COW is fine, as COW never overwrites extents in commit tree.
+ */
+ if (sctx->is_dev_replace) {
+ btrfs_wait_nocow_writers(cache);
+ btrfs_wait_ordered_roots(fs_info, U64_MAX, cache->start,
+ cache->length);
+ }
+
+ scrub_pause_off(fs_info);
down_write(&dev_replace->rwsem);
dev_replace->cursor_right = found_key.offset + length;
dev_replace->cursor_left = found_key.offset;