Re: [PATCH] btrfs: scrub: skip PREALLOC extents on RAID stripe-tree

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

在 2024/9/13 00:03, Johannes Thumshirn 写道:
> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>
> When scrubbing a RAID stripe-tree based filesystem with preallocated
> extents, the btrfs_map_block() called by
> scrub_submit_extent_sector_read() will return ENOENT, because there is
> no RAID stripe-tree entry for preallocated extents. This then causes
> the sector to be marked as a sector with an I/O error.
>
> To prevent this false alert don't mark secotors for that
> btrfs_map_block() returned an ENOENT as I/O errors but skip them.
>
> This results for example in errors in fstests' btrfs/060 .. btrfs/074
> which all perform fsstress and scrub operations. Whit this fix, these
> errors are gone and the tests pass again.
>
> Cc: Qu Wenru <wqu@suse.com>

My concern is, ENOENT can be some real problems other than PREALLOC.
I'd prefer this to be the last-resort method.

Would it be possible to create an RST entry for preallocated operations
manually? E.g. without creating a dummy OE, but just insert the needed
RST entries into RST tree at fallocate time?

Thanks,
Qu

> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>   fs/btrfs/scrub.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index 3a3427428074..b195c41676e3 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -1657,7 +1657,7 @@ static u32 stripe_length(const struct scrub_stripe *stripe)
>   }
>
>   static void scrub_submit_extent_sector_read(struct scrub_ctx *sctx,
> -					    struct scrub_stripe *stripe)
> +					   struct scrub_stripe *stripe)
>   {
>   	struct btrfs_fs_info *fs_info = stripe->bg->fs_info;
>   	struct btrfs_bio *bbio = NULL;
> @@ -1703,10 +1703,21 @@ static void scrub_submit_extent_sector_read(struct scrub_ctx *sctx,
>   			err = btrfs_map_block(fs_info, BTRFS_MAP_READ, logical,
>   					      &stripe_len, &bioc, &io_stripe, &mirror);
>   			btrfs_put_bioc(bioc);
> -			if (err < 0) {
> +			if (err < 0 && err != -ENOENT) {
>   				set_bit(i, &stripe->io_error_bitmap);
>   				set_bit(i, &stripe->error_bitmap);
>   				continue;
> +			} else if (err == -ENOENT) {
> +				/*
> +				 * btrfs_map_block() returns -ENOENT if it can't
> +				 * find the logical  address in the RAID stripe
> +				 * tree. This can happens on PREALLOC  extents.
> +				 * As we know the extent tree  has an extent
> +				 * recorded there, we can be sure this is a
> +				 * PREALLOC  extent, so skip this sector and
> +				 * continue to the next.
> +				 */
> +				continue;
>   			}
>
>   			bbio = btrfs_bio_alloc(stripe->nr_sectors, REQ_OP_READ,