Re: [PATCH 03/34] btrfs: add a btrfs_inode pointer to struct btrfs_bio

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On Tue, Mar 07, 2023 at 09:44:32AM +0800, Qu Wenruo wrote:
> With my recent restart on scrub rework, this patch makes me wonder, what if 
> scrub wants to use btrfs_bio, but don't want to pass a valid btrfs_inode 
> pointer?

The full inode is only really needed for the data repair code.  But a lot
of code uses the fs_info, which would have to be added as a separate
counter.  The other usage is the sync_writers counter, which is a bit
odd and should probably be keyed off the REQ_SYNC flag instead.

> E.g. scrub code just wants to read certain mirror of a logical bytenr.
> This can simplify the handling of RAID56, as for data stripes the repair 
> path is the same, just try the next mirror(s).
>
> Furthermore most of the new btrfs_bio code is handling data reads by 
> triggering read-repair automatically.
> This can be unnecessary for scrub.

This sounds like you don't want to use the btrfs_bio at all as you
don't rely on any of the functionality from it.

>
> And since we're here, can we also have btrfs equivalent of on-stack bio?
> As scrub can benefit a lot from that, as for sector-by-sector read, we want 
> to avoid repeating allocating/freeing a btrfs_bio just for reading one 
> sector.
> (The existing behavior is using on-stack bio with bio_init/bio_uninit 
> inside scrub_recheck_block())

You can do that right now by declaring a btrfs_bio on-stack and then
calling bio_init on the embedded bio followed by a btrfs_bio_init on
the btrfs_bio.  But I don't think doing this will actually be a win
for the scrub code in terms of speed or code size.



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