Re: [PATCH 03/10] btrfs: split btrfs_submit_data_bio

[Qu Wenruo <wqu@suse.com>]

On 2022/4/25 19:34, Christoph Hellwig wrote:
> On Mon, Apr 25, 2022 at 07:31:08PM +0800, Qu Wenruo wrote:
>> Then it comes against the btrfs read time repair.
>>
>> Currently we split bio to make sure we never need to split bio at
>> btrfs_map_bio() time.
>>
>> But this is against common layer separation.
>>
>> And we really want the ability to read a partially corrupted bio (some
>> part matches csum, some doesn't), no matter if the bio is cloned or not.
>>
>> Especially, we already have cloned bio which needs repair (for dio).
> 
> I have a barely working version based on your patches to split the
> bio in btrfs_bio_map that solves this problem.  But the next step
> only removed the save iter for writes, where the only user is
> index_one_bio.  And the fix for that is pretty trivial :)

That's only for RAID56, aren't you going to remove btrfs_bio usage 
completely for all write (including buffered, non-compressing write)?

Thanks,
Qu
> 
> ---
>  From c8fe61748ebc583a7f57c8e5de79f92428e5717c Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@lst.de>
> Date: Mon, 25 Apr 2022 13:23:54 +0200
> Subject: btrfs: stop looking at btrfs_bio->iter in index_one_bio
> 
> All the bios that index_one_bio operates on are the bios submitted by the
> upper layer.  These are never resubmitted to an actual device by the
> raid56 code, and thus the iter never changes from the initial state.
> Thus we can always just use bi_iter directly as it will be the same as
> the saved copy.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   fs/btrfs/raid56.c | 3 ---
>   1 file changed, 3 deletions(-)
> 
> diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> index 1a3c1a9b10d0b..8b40353bb89db 100644
> --- a/fs/btrfs/raid56.c
> +++ b/fs/btrfs/raid56.c
> @@ -1218,9 +1218,6 @@ static void index_one_bio(struct btrfs_raid_bio *rbio, struct bio *bio)
>   	u32 offset = (bio->bi_iter.bi_sector << SECTOR_SHIFT) -
>   		     rbio->bioc->raid_map[0];
>   
> -	if (bio_flagged(bio, BIO_CLONED))
> -		bio->bi_iter = btrfs_bio(bio)->iter;
> -
>   	bio_for_each_segment(bvec, bio, iter) {
>   		u32 bvec_offset;
>