Re: [PATCH] btrfs: Fix and enhance merge_extent_mapping() to insert best fitted extent map

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Sep 17, 2014 at 11:53:35AM +0800, Qu Wenruo wrote:
> The following commit enhanced the merge_extent_mapping() to reduce
> fragment in extent map tree, but it can't handle case which existing
> lies before map_start:
> 51f39 btrfs: Use right extent length when inserting overlap extent map.
> 
> [BUG]
> When existing extent map's start is before map_start,
> the em->len will be minus, which will corrupt the extent map and fail to
> insert the new extent map.
> This will happen when someone get a large extent map, but when it is
> going to insert it into extent map tree, some one has already commit
> some write and split the huge extent into small parts.
> 
> [REPRODUCER]
> It is very easy to tiger using filebench with randomrw personality.
> It is about 100% to reproduce when using 8G preallocated file in 60s
> randonrw test.
> 
> [FIX]
> This patch can now handle any existing extent position.
> Since it does not directly use existing->start, now it will find the
> previous and next extent around map_start.
> So the old existing->start < map_start bug will never happen again.
> 
> [ENHANCE]
> This patch will insert the best fitted extent map into extent map tree,
> other than the oldest [map_start, map_start + sectorsize) or the
> relatively newer but not perfect [map_start, existing->start).
> 
> The patch will first search existing extent that does not intersects with
> the desired map range [map_start, map_start + len).
> The existing extent will be either before or behind map_start, and based
> on the existing extent, we can find out the previous and next extent
> around map_start.
> 
> So the best fitted extent would be [prev->end, next->start).
> For prev or next is not found, em->start would be prev->end and em->end
> wold be next->start.
> 
> With this patch, the fragment in extent map tree should be reduced much
> more than the 51f39 commit and reduce an unneeded extent map tree search.
> 
> Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
> ---
>  fs/btrfs/inode.c | 79 ++++++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 57 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 016c403..8039021 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -6191,21 +6191,60 @@ out_fail_inode:
>  	goto out_fail;
>  }
>  
> +/* Find next extent map of a given extent map, caller needs to ensure locks */
> +static struct extent_map *next_extent_map(struct extent_map *em)
> +{
> +	struct rb_node *next;
> +
> +	next = rb_next(&em->rb_node);
> +	if (!next)
> +		return NULL;
> +	return container_of(next, struct extent_map, rb_node);
> +}
> +
> +static struct extent_map *prev_extent_map(struct extent_map *em)
> +{
> +	struct rb_node *prev;
> +
> +	prev = rb_prev(&em->rb_node);
> +	if (!prev)
> +		return NULL;
> +	return container_of(prev, struct extent_map, rb_node);
> +}
> +
>  /* helper for btfs_get_extent.  Given an existing extent in the tree,
> + * the existing extent is the nearest extent to map_start,
>   * and an extent that you want to insert, deal with overlap and insert
> - * the new extent into the tree.
> + * the best fitted new extent into the tree.
>   */
>  static int merge_extent_mapping(struct extent_map_tree *em_tree,
>  				struct extent_map *existing,
>  				struct extent_map *em,
>  				u64 map_start)
>  {
> +	struct extent_map *prev;
> +	struct extent_map *next;
> +	u64 start;
> +	u64 end;
>  	u64 start_diff;
>  
>  	BUG_ON(map_start < em->start || map_start >= extent_map_end(em));
> -	start_diff = map_start - em->start;
> -	em->start = map_start;
> -	em->len = existing->start - em->start;
> +
> +	if (existing->start > map_start) {
> +		next = existing;
> +		prev = prev_extent_map(next);
> +	} else {
> +		prev = existing;
> +		next = next_extent_map(prev);
> +	}
> +
> +	start = prev ? extent_map_end(prev) : em->start;
> +	start = max_t(u64, start, em->start);
> +	end = next ? next->start : extent_map_end(em);
> +	end = min_t(u64, end, extent_map_end(em));
> +	start_diff = start - em->start;
> +	em->start = start;
> +	em->len = end - start;
>  	if (em->block_start < EXTENT_MAP_LAST_BYTE &&
>  	    !test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
>  		em->block_start += start_diff;
> @@ -6482,25 +6521,21 @@ insert:
>  
>  		ret = 0;
>  
> -		existing = lookup_extent_mapping(em_tree, start, len);
> -		if (existing && (existing->start > start ||
> -		    existing->start + existing->len <= start)) {
> +		existing = search_extent_mapping(em_tree, start, len);
> +		/*
> +		 * existing will always be non-NULL, since there must be
> +		 * extent causing the -EEXIST.
> +		 */
> +		if (start >= extent_map_end(existing) ||
> +		    start + len <= existing->start) {

This will introduce something wrong, the 'else' part is 'em = existing;',
and the condition is actually
(start < extent_map_end(existing) && start + len > existing->start),
which means the existing overlaps with [start, start+len).

And one of overlapping cases is (existing->start > start), ie. em->start > start, this is
against our rule of btrfs_get_extent,

struct extent_map *btrfs_get_extent(...)
{
	[...]
	insert:                                                                          
		btrfs_release_path(path);                                                
		if (em->start > start || extent_map_end(em) <= start) {                  
			btrfs_err(root->fs_info, "bad extent! em: [%llu %llu] passed
	[%llu %llu]",
				em->start, em->len, start, len);                         
			err = -EIO;                                                      
                goto out;                                                        
        }                                                                        
        [...]
}

thanks,
-liubo

> +			/*
> +			 * The existing extent map is the one nearest to
> +			 * the [start, start + len) range which overlaps
> +			 */
> +			err = merge_extent_mapping(em_tree, existing,
> +						   em, start);
>  			free_extent_map(existing);
> -			existing = NULL;
> -		}
> -		if (!existing) {
> -			existing = lookup_extent_mapping(em_tree, em->start,
> -							 em->len);
> -			if (existing) {
> -				err = merge_extent_mapping(em_tree, existing,
> -							   em, start);
> -				free_extent_map(existing);
> -				if (err) {
> -					free_extent_map(em);
> -					em = NULL;
> -				}
> -			} else {
> -				err = -EIO;
> +			if (err) {
>  				free_extent_map(em);
>  				em = NULL;
>  			}

> -- 
> 2.1.0
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html