Re: [PATCH] Btrfs: send, skip backreference walking for extents with many references

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/10/30 下午8:23, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Backreference walking, which is used by send to figure if it can issue
> clone operations instead of write operations, can be very slow and use too
> much memory when extents have many references. This change simply skips
> backreference walking when an extent has more than 64 references, in which
> case we fallback to a write operation instead of a clone operation. This
> limit is conservative and in practice I observed no signicant slowdown
> with up to 100 references and still low memory usage up to that limit.
> 
> This is a temporary workaround until there are speedups in the backref
> walking code, and as such it does not attempt to add extra interfaces or
> knobs to tweak the threshold.
> 
> Reported-by: Atemu <atemu.main@gmail.com>
> Link: https://lore.kernel.org/linux-btrfs/CAE4GHgkvqVADtS4AzcQJxo0Q1jKQgKaW3JGp3SGdoinVo=C9eQ@mail.gmail.com/T/#me55dc0987f9cc2acaa54372ce0492c65782be3fa
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

The workaround is much better than the old
completely-disable-reflink-detection one.

Thanks,
Qu

> ---
>  fs/btrfs/send.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> index 123ac54af071..518ec1265a0c 100644
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -25,6 +25,14 @@
>  #include "compression.h"
>  
>  /*
> + * Maximum number of references an extent can have in order for us to attempt to
> + * issue clone operations instead of write operations. This currently exists to
> + * avoid hitting limitations of the backreference walking code (taking a lot of
> + * time and using too much memory for extents with large number of references).
> + */
> +#define SEND_MAX_EXTENT_REFS	64
> +
> +/*
>   * A fs_path is a helper to dynamically build path names with unknown size.
>   * It reallocates the internal buffer on demand.
>   * It allows fast adding of path elements on the right side (normal path) and
> @@ -1302,6 +1310,7 @@ static int find_extent_clone(struct send_ctx *sctx,
>  	struct clone_root *cur_clone_root;
>  	struct btrfs_key found_key;
>  	struct btrfs_path *tmp_path;
> +	struct btrfs_extent_item *ei;
>  	int compressed;
>  	u32 i;
>  
> @@ -1349,7 +1358,6 @@ static int find_extent_clone(struct send_ctx *sctx,
>  	ret = extent_from_logical(fs_info, disk_byte, tmp_path,
>  				  &found_key, &flags);
>  	up_read(&fs_info->commit_root_sem);
> -	btrfs_release_path(tmp_path);
>  
>  	if (ret < 0)
>  		goto out;
> @@ -1358,6 +1366,21 @@ static int find_extent_clone(struct send_ctx *sctx,
>  		goto out;
>  	}
>  
> +	ei = btrfs_item_ptr(tmp_path->nodes[0], tmp_path->slots[0],
> +			    struct btrfs_extent_item);
> +	/*
> +	 * Backreference walking (iterate_extent_inodes() below) is currently
> +	 * too expensive when an extent has a large number of references, both
> +	 * in time spent and used memory. So for now just fallback to write
> +	 * operations instead of clone operations when an extent has more than
> +	 * a certain amount of references.
> +	 */
> +	if (btrfs_extent_refs(tmp_path->nodes[0], ei) > SEND_MAX_EXTENT_REFS) {
> +		ret = -ENOENT;
> +		goto out;
> +	}
> +	btrfs_release_path(tmp_path);
> +
>  	/*
>  	 * Setup the clone roots.
>  	 */
> 


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