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
<transaction N starts>
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 <fdmanana@suse.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
This commit is contained in:
Filipe Manana
parent
57ba4cb85b
commit
f0e9b7d640
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@ -718,12 +718,13 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr,
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return count;
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}
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void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
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int btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
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const u64 range_start, const u64 range_len)
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{
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struct btrfs_root *root;
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struct list_head splice;
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int done;
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int total_done = 0;
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INIT_LIST_HEAD(&splice);
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@ -742,6 +743,7 @@ void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
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done = btrfs_wait_ordered_extents(root, nr,
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range_start, range_len);
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btrfs_put_fs_root(root);
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total_done += done;
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spin_lock(&fs_info->ordered_root_lock);
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if (nr != -1) {
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@ -752,6 +754,8 @@ void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
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list_splice_tail(&splice, &fs_info->ordered_roots);
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spin_unlock(&fs_info->ordered_root_lock);
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mutex_unlock(&fs_info->ordered_operations_mutex);
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return total_done;
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}
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/*
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@ -199,7 +199,7 @@ int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr,
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u32 *sum, int len);
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int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr,
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const u64 range_start, const u64 range_len);
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void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
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int btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
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const u64 range_start, const u64 range_len);
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void btrfs_get_logged_extents(struct inode *inode,
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struct list_head *logged_list,
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@ -3582,6 +3582,46 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
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*/
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scrub_pause_on(fs_info);
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ret = btrfs_inc_block_group_ro(root, cache);
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if (!ret && is_dev_replace) {
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/*
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* If we are doing a device replace wait for any tasks
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* that started dellaloc right before we set the block
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* group to RO mode, as they might have just allocated
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* an extent from it or decided they could do a nocow
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* write. And if any such tasks did that, wait for their
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* ordered extents to complete and then commit the
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* current transaction, so that we can later see the new
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* extent items in the extent tree - the ordered extents
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* create delayed data references (for cow writes) when
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* they complete, which will be run and insert the
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* corresponding extent items into the extent tree when
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* we commit the transaction they used when running
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* inode.c:btrfs_finish_ordered_io(). We later use
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* the commit root of the extent tree to find extents
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* to copy from the srcdev into the tgtdev, and we don't
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* want to miss any new extents.
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*/
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btrfs_wait_block_group_reservations(cache);
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btrfs_wait_nocow_writers(cache);
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ret = btrfs_wait_ordered_roots(fs_info, -1,
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cache->key.objectid,
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cache->key.offset);
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if (ret > 0) {
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struct btrfs_trans_handle *trans;
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trans = btrfs_join_transaction(root);
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if (IS_ERR(trans))
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ret = PTR_ERR(trans);
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else
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ret = btrfs_commit_transaction(trans,
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root);
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if (ret) {
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scrub_pause_off(fs_info);
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btrfs_put_block_group(cache);
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break;
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}
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}
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}
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scrub_pause_off(fs_info);
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if (ret == 0) {
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