Re: [PATCH 02/19] btrfs: handle checksum validation and repair at the storage layer

From: David Sterba <dsterba@suse.cz>
To: Christoph Hellwig <hch@lst.de>
Cc: Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Naohiro Aota <naohiro.aota@wdc.com>,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>,
	Qu Wenruo <wqu@suse.com>, Jens Axboe <axboe@kernel.dk>,
	"Darrick J. Wong" <djwong@kernel.org>,
	linux-block@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 02/19] btrfs: handle checksum validation and repair at the storage layer
Date: Mon, 12 Dec 2022 23:13:47 +0100	[thread overview]
Message-ID: <20221212221347.GC5824@twin.jikos.cz> (raw)
In-Reply-To: <20221120124734.18634-3-hch@lst.de>

On Sun, Nov 20, 2022 at 01:47:17PM +0100, Christoph Hellwig wrote:
> Currently btrfs handles checksum validation and repair in the end I/O
> handler for the btrfs_bio.  This leads to a lot of duplicate code
> plus issues with variying semantics or bugs, e.g.
> 
>  - the until recently broken repair for compressed extents
>  - the fact that encoded reads validate the checksums but do not kick
>    of read repair
>  - the inconsistent checking of the BTRFS_FS_STATE_NO_CSUMS flag
> 
> This commit revamps the checksum validation and repair code to instead
> work below the btrfs_submit_bio interfaces.  For this to work we need
> to make sure an inode is available, so that is added as a parameter
> to btrfs_bio_alloc.  With that btrfs_submit_bio can preload
> btrfs_bio.csum from the csum tree without help from the upper layers,
> and the low-level I/O completion can iterate over the bio and verify
> the checksums.
> 
> In case of a checksum failure (or a plain old I/O error), the repair
> is now kicked off before the upper level ->end_io handler is invoked.
> Tracking of the repair status is massively simplified by just keeping
> a small failed_bio structure per bio with failed sectors and otherwise
> using the information in the repair bio.  The per-inode I/O failure
> tree can be entirely removed.
> 
> The saved bvec_iter in the btrfs_bio is now competely managed by
> btrfs_submit_bio and must not be accessed by the callers.
> 
> There is one significant behavior change here:  If repair fails or
> is impossible to start with, the whole bio will be failed to the
> upper layer.  This is the behavior that all I/O submitters execept
> for buffered I/O already emulated in their end_io handler.  For
> buffered I/O this now means that a large readahead request can
> fail due to a single bad sector, but as readahead errors are igored
> the following readpage if the sector is actually accessed will
> still be able to read.  This also matches the I/O failure handling
> in other file systems.

This patch is apparently doing several things at once, please split it.
Thanks.