Re: [PATCH RFC v2 02/12] btrfs: always save bio::bi_iter into btrfs_bio::iter before submitting

[Qu Wenruo <wqu@suse.com>]

On 2022/4/27 15:18, Qu Wenruo wrote:
> Lower level bio mapping, from driver to dm, even btrfs chunk mapping
> can modify bio::bi_iter.
> 
> This prevents us from doing two things:
> 
> - Iterate the bio range of a cloned bio
>    This is only utilized by direct IO, thus it's already using
>    btrfs_bio::iter, which is populated in btrfs_bio_clone_partial().
> 
> - Grab the original logical bytenr of a bio
>    This will be utilized by incoming read repair patches.
> 
> So to make sure all btrfs_bio submitted to own a proper iter, this patch
> will assigned btrfs_bio::iter in the following call sites:
> 
> - btrfs_submit_dio_bio()
> - submit_one_bio()
> - submit_compressed_bio()
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Cc Christoph, this is definitely going to conflicts with his further 
cleanups.

Maybe I can update this patch to only set the iter for read bios?

Thanks,
Qu

> ---
>   fs/btrfs/compression.c | 2 ++
>   fs/btrfs/extent_io.c   | 6 ++++++
>   fs/btrfs/inode.c       | 2 ++
>   3 files changed, 10 insertions(+)
> 
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index 69c060dc024c..559bf53beaed 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -428,6 +428,8 @@ static blk_status_t submit_compressed_bio(struct btrfs_fs_info *fs_info,
>   	blk_status_t ret;
>   
>   	ASSERT(bio->bi_iter.bi_size);
> +	/* Check submit_one_bio() for the reason */
> +	btrfs_bio(bio)->iter = bio->bi_iter;
>   	ret = btrfs_bio_wq_end_io(fs_info, bio, BTRFS_WQ_ENDIO_DATA);
>   	if (ret)
>   		return ret;
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 07888cce3bce..a3962df30603 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -186,6 +186,12 @@ static void submit_one_bio(struct bio *bio, int mirror_num, unsigned long bio_fl
>   	/* Caller should ensure the bio has at least some range added */
>   	ASSERT(bio->bi_iter.bi_size);
>   
> +	/*
> +	 * Save current bi_iter into btrfs_bio::iter, as lower level (including
> +	 * btrfs chunk mapping) can modify bi_iter, preventing us from using
> +	 * bi_iter to iterate cloned bio or grab the original logical bytenr.
> +	 */
> +	btrfs_bio(bio)->iter = bio->bi_iter;
>   	if (is_data_inode(tree->private_data))
>   		btrfs_submit_data_bio(tree->private_data, bio, mirror_num,
>   					    bio_flags);
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index b4cfd78f50e8..1b596de0c4e9 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -7923,6 +7923,8 @@ static inline blk_status_t btrfs_submit_dio_bio(struct bio *bio,
>   	bool write = btrfs_op(bio) == BTRFS_MAP_WRITE;
>   	blk_status_t ret;
>   
> +	btrfs_bio(bio)->iter = bio->bi_iter;
> +
>   	/* Check btrfs_submit_bio_hook() for rules about async submit. */
>   	if (async_submit)
>   		async_submit = !atomic_read(&BTRFS_I(inode)->sync_writers);