Re: [PATCH] btrfs: properly split extent_map for REQ_OP_ZONE_APPEND

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2021/06/28 20:50, Qu Wenruo wrote:
> 
> 
> On 2021/6/28 下午4:57, Naohiro Aota wrote:
>> Damien reported a test failure with btrfs/209. The test itself ran fine,
>> but the fsck run afterwards reported a corrupted filesystem.
>>
>> The filesystem corruption happens because we're splitting an extent and
>> then writing the extent twice. We have to split the extent though, because
>> we're creating too large extents for a REQ_OP_ZONE_APPEND operation.
>>
>> When dumping the extent tree, we can see two EXTENT_ITEMs at the same
>> start address but different lengths.
>>
>> $ btrfs inspect dump-tree /dev/nullb1 -t extent
>> ...
>>     item 19 key (269484032 EXTENT_ITEM 126976) itemoff 15470 itemsize 53
>>             refs 1 gen 7 flags DATA
>>             extent data backref root FS_TREE objectid 257 offset 786432 count 1
>>     item 20 key (269484032 EXTENT_ITEM 262144) itemoff 15417 itemsize 53
>>             refs 1 gen 7 flags DATA
>>             extent data backref root FS_TREE objectid 257 offset 786432 count 1
>>
>> The duplicated EXTENT_ITEMs originally come from wrongly split extent_map in
>> extract_ordered_extent(). Since extract_ordered_extent() uses
>> create_io_em() to split an existing extent_map, we will have
>> split->orig_start != split->start. Then, it will be logged with non-zero
>> "extent data offset". Finally, the logged entries are replayed into
>> a duplicated EXTENT_ITEM.
>>
>> Introduce and use proper splitting function for extent_map. The function is
>> intended to be simple and specific usage for extract_ordered_extent() e.g.
>> not supporting compression case (we do not allow splitting compressed
>> extent_map anyway).
> 
> This may be a pretty stupid question, but why do we need to split the
> extent map (and extent item) into several more and causing more extent
> items?
> 
> 
> I understand for zoned write, we have extra limitation on how many bytes
> we can submit before reaching the zone limit.
> 
> But we also have stripe boundary for non-zoned device.
> 
> And in that case, we just split them into different bios, other than
> split the extent into smaller extents.
> 
> Of course for current zoned support, only SINGLE profile is supported
> thus no stripe boundary to bother.
> 
> But I'm wondering if we could do the same thing without really splitting
> the extent map.

The problem is not the limit on the zone end, which as you mention is the same
as the block group end. The problem is that data write use zone append (ZA)
operations. ZA BIOs cannot be split so a large extent may need to be processed
with multiple ZA BIOs, While that is also true for regular writes, the major
difference is that ZA are "nameless" write operation giving back the written
sectors on completion. And ZA operations may be reordered by the block layer
(not intentionally though). Combine both of these characteristics and you can
see that the data for a large extent may end up being shuffled when written
resulting in data corruption and the impossibility to map the extent to some
start sector.

To avoid this problem, zoned btrfs uses the principle "one data extent == one ZA
BIO". So large extents need to be split. This is unfortunate, but we can revisit
this later and optimize, e.g. merge back together the fragments of an extent
once written if they actually were written sequentially in the zone.

> 
> Thanks,
> Qu
> 
>>
>> Fixes: d22002fd37bd ("btrfs: zoned: split ordered extent when bio is sent")
>> Cc: stable@vger.kernel.org # 5.12+
>> Reported-by: Damien Le Moal <damien.lemoal@wdc.com>
>> Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
>> ---
>>   fs/btrfs/inode.c | 151 ++++++++++++++++++++++++++++++++++++++---------
>>   1 file changed, 122 insertions(+), 29 deletions(-)
>>
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index e6eb20987351..79cdcaeab8de 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -2271,13 +2271,131 @@ static blk_status_t btrfs_submit_bio_start(struct inode *inode, struct bio *bio,
>>   	return btrfs_csum_one_bio(BTRFS_I(inode), bio, 0, 0);
>>   }
>>
>> +/*
>> + * split_zoned_em - split an extent_map at [start, start+len]
>> + *
>> + * This function is intended to be used only for extract_ordered_extent().
>> + */
>> +static int split_zoned_em(struct btrfs_inode *inode, u64 start, u64 len,
>> +			  u64 pre, u64 post)
>> +{
>> +	struct extent_map_tree *em_tree = &inode->extent_tree;
>> +	struct extent_map *em;
>> +	struct extent_map *split_pre = NULL;
>> +	struct extent_map *split_mid = NULL;
>> +	struct extent_map *split_post = NULL;
>> +	int ret = 0;
>> +	int modified;
>> +	unsigned long flags;
>> +
>> +	/* Sanity check */
>> +	if (pre == 0 && post == 0)
>> +		return 0;
>> +
>> +	split_pre = alloc_extent_map();
>> +	if (pre)
>> +		split_mid = alloc_extent_map();
>> +	if (post)
>> +		split_post = alloc_extent_map();
>> +	if (!split_pre || (pre && !split_mid) || (post && !split_post)) {
>> +		ret = -ENOMEM;
>> +		goto out;
>> +	}
>> +
>> +	ASSERT(pre + post < len);
>> +
>> +	lock_extent(&inode->io_tree, start, start + len - 1);
>> +	write_lock(&em_tree->lock);
>> +	em = lookup_extent_mapping(em_tree, start, len);
>> +	if (!em) {
>> +		ret = -EIO;
>> +		goto out_unlock;
>> +	}
>> +
>> +	ASSERT(em->len == len);
>> +	ASSERT(!test_bit(EXTENT_FLAG_COMPRESSED, &em->flags));
>> +	ASSERT(em->block_start < EXTENT_MAP_LAST_BYTE);
>> +
>> +	flags = em->flags;
>> +	clear_bit(EXTENT_FLAG_PINNED, &em->flags);
>> +	clear_bit(EXTENT_FLAG_LOGGING, &flags);
>> +	modified = !list_empty(&em->list);
>> +
>> +	/*
>> +	 * First, replace the em with a new extent_map starting from
>> +	 * em->start
>> +	 */
>> +
>> +	split_pre->start = em->start;
>> +	split_pre->len = pre ? pre : (em->len - post);
>> +	split_pre->orig_start = split_pre->start;
>> +	split_pre->block_start = em->block_start;
>> +	split_pre->block_len = split_pre->len;
>> +	split_pre->orig_block_len = split_pre->block_len;
>> +	split_pre->ram_bytes = split_pre->len;
>> +	split_pre->flags = flags;
>> +	split_pre->compress_type = em->compress_type;
>> +	split_pre->generation = em->generation;
>> +
>> +	replace_extent_mapping(em_tree, em, split_pre, modified);
>> +
>> +	/*
>> +	 * Now we only have an extent_map at:
>> +	 *     [em->start, em->start + pre] if pre != 0
>> +	 *     [em->start, em->start + em->len - post] if pre == 0
>> +	 */
>> +
>> +	if (pre) {
>> +		/* Insert the middle extent_map */
>> +		split_mid->start = em->start + pre;
>> +		split_mid->len = em->len - pre - post;
>> +		split_mid->orig_start = split_mid->start;
>> +		split_mid->block_start = em->block_start + pre;
>> +		split_mid->block_len = split_mid->len;
>> +		split_mid->orig_block_len = split_mid->block_len;
>> +		split_mid->ram_bytes = split_mid->len;
>> +		split_mid->flags = flags;
>> +		split_mid->compress_type = em->compress_type;
>> +		split_mid->generation = em->generation;
>> +		add_extent_mapping(em_tree, split_mid, modified);
>> +	}
>> +
>> +	if (post) {
>> +		split_post->start = em->start + em->len - post;
>> +		split_post->len = post;
>> +		split_post->orig_start = split_post->start;
>> +		split_post->block_start = em->block_start + em->len - post;
>> +		split_post->block_len = split_post->len;
>> +		split_post->orig_block_len = split_post->block_len;
>> +		split_post->ram_bytes = split_post->len;
>> +		split_post->flags = flags;
>> +		split_post->compress_type = em->compress_type;
>> +		split_post->generation = em->generation;
>> +		add_extent_mapping(em_tree, split_post, modified);
>> +	}
>> +
>> +	/* once for us */
>> +	free_extent_map(em);
>> +	/* once for the tree */
>> +	free_extent_map(em);
>> +
>> +out_unlock:
>> +	write_unlock(&em_tree->lock);
>> +	unlock_extent(&inode->io_tree, start, start + len - 1);
>> +out:
>> +	free_extent_map(split_pre);
>> +	free_extent_map(split_mid);
>> +	free_extent_map(split_post);
>> +
>> +	return ret;
>> +}
>> +
>>   static blk_status_t extract_ordered_extent(struct btrfs_inode *inode,
>>   					   struct bio *bio, loff_t file_offset)
>>   {
>>   	struct btrfs_ordered_extent *ordered;
>> -	struct extent_map *em = NULL, *em_new = NULL;
>> -	struct extent_map_tree *em_tree = &inode->extent_tree;
>>   	u64 start = (u64)bio->bi_iter.bi_sector << SECTOR_SHIFT;
>> +	u64 file_len;
>>   	u64 len = bio->bi_iter.bi_size;
>>   	u64 end = start + len;
>>   	u64 ordered_end;
>> @@ -2317,41 +2435,16 @@ static blk_status_t extract_ordered_extent(struct btrfs_inode *inode,
>>   		goto out;
>>   	}
>>
>> +	file_len = ordered->num_bytes;
>>   	pre = start - ordered->disk_bytenr;
>>   	post = ordered_end - end;
>>
>>   	ret = btrfs_split_ordered_extent(ordered, pre, post);
>>   	if (ret)
>>   		goto out;
>> -
>> -	read_lock(&em_tree->lock);
>> -	em = lookup_extent_mapping(em_tree, ordered->file_offset, len);
>> -	if (!em) {
>> -		read_unlock(&em_tree->lock);
>> -		ret = -EIO;
>> -		goto out;
>> -	}
>> -	read_unlock(&em_tree->lock);
>> -
>> -	ASSERT(!test_bit(EXTENT_FLAG_COMPRESSED, &em->flags));
>> -	/*
>> -	 * We cannot reuse em_new here but have to create a new one, as
>> -	 * unpin_extent_cache() expects the start of the extent map to be the
>> -	 * logical offset of the file, which does not hold true anymore after
>> -	 * splitting.
>> -	 */
>> -	em_new = create_io_em(inode, em->start + pre, len,
>> -			      em->start + pre, em->block_start + pre, len,
>> -			      len, len, BTRFS_COMPRESS_NONE,
>> -			      BTRFS_ORDERED_REGULAR);
>> -	if (IS_ERR(em_new)) {
>> -		ret = PTR_ERR(em_new);
>> -		goto out;
>> -	}
>> -	free_extent_map(em_new);
>> +	ret = split_zoned_em(inode, file_offset, file_len, pre, post);
>>
>>   out:
>> -	free_extent_map(em);
>>   	btrfs_put_ordered_extent(ordered);
>>
>>   	return errno_to_blk_status(ret);
>>
> 


-- 
Damien Le Moal
Western Digital Research