Re: [PATCH v2 2/2] btrfs: raid56: pass REQ_META to per-device bios

[Qu Wenruo]

在 2026/11/7 04:03, Mark Korenberg via B4 Relay 写道:
> From: Mark Korenberg <socketpair@xxxxxxxxx>
>
> RAID5/6 writes, and reads that need a rebuild, go through an rbio,
> which builds new per-device bios from a bare REQ_OP_READ or
> REQ_OP_WRITE.  So all tree block writes on RAID5/6 lose REQ_META, while
> on the other profiles the per-device bios are clones and keep it.  The
> same goes for a tree block read rebuilt from the other stripes, for
> example with a missing device.
>
> Pass REQ_META on to all per-device bios of an rbio, data and P/Q alike,
> if any bio it serves has it.  Scrub and relocation do not set REQ_META,
> so their rbios stay untagged, as do rbios that only serve file data.
>
> Signed-off-by: Mark Korenberg <socketpair@xxxxxxxxx>

NACK.

One should not even use RAID56 for metadata chunks in the first place.
Nor we're any different from other RAID56 implementation.

> ---
>   fs/btrfs/raid56.c | 14 +++++++++++---
>   fs/btrfs/raid56.h |  7 +++++++
>   2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> index a8a8004bb14c..63b5acbf107e 100644
> --- a/fs/btrfs/raid56.c
> +++ b/fs/btrfs/raid56.c
> @@ -1202,10 +1202,15 @@ static int rbio_add_io_paddrs(struct btrfs_raid_bio *rbio, struct bio_list *bio_
>   		}
>   	}
>   
> -	/* put a new bio on the list */
> +	/*
> +	 * Put a new bio on the list.  Every per-device bio, data and P/Q
> +	 * alike, serves the bios in rbio->bio_list, so it is metadata I/O if
> +	 * any of them is: a tree block write, or a tree block read rebuilt
> +	 * from the other stripes.
> +	 */
>   	bio = bio_alloc(stripe->dev->bdev,
>   			max(rbio->bioc->stripe_len >> PAGE_SHIFT, 1),
> -			op, GFP_NOFS);
> +			op | rbio->req_meta, GFP_NOFS);
>   	bio->bi_iter.bi_sector = disk_start >> SECTOR_SHIFT;
>   	bio->bi_private = rbio;
>   
> @@ -1246,8 +1251,11 @@ static void index_rbio_pages(struct btrfs_raid_bio *rbio)
>   	struct bio *bio;
>   
>   	spin_lock(&rbio->bio_list_lock);
> -	bio_list_for_each(bio, &rbio->bio_list)
> +	rbio->req_meta = 0;
> +	bio_list_for_each(bio, &rbio->bio_list) {
>   		index_one_bio(rbio, bio);
> +		rbio->req_meta |= bio->bi_opf & REQ_META;
> +	}
>   
>   	spin_unlock(&rbio->bio_list_lock);
>   }
> diff --git a/fs/btrfs/raid56.h b/fs/btrfs/raid56.h
> index 8700910b8dc9..a5a0ff8e4a03 100644
> --- a/fs/btrfs/raid56.h
> +++ b/fs/btrfs/raid56.h
> @@ -139,6 +139,13 @@ struct btrfs_raid_bio {
>   	 */
>   	enum btrfs_rbio_ops operation;
>   
> +	/*
> +	 * REQ_META if any bio in bio_list has it.  Set under bio_list_lock by
> +	 * index_rbio_pages(), which runs before every assembly of per-device
> +	 * bios.
> +	 */
> +	blk_opf_t req_meta;
> +
>   	/* How many pages there are for the full stripe including P/Q */
>   	u16 nr_pages;
>