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Date:   Wed, 5 Apr 2023 08:06:27 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Andrey Albershteyn <aalbersh@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>, dchinner@redhat.com,
        ebiggers@kernel.org, linux-xfs@vger.kernel.org,
        fsverity@lists.linux.dev, rpeterso@redhat.com, agruenba@redhat.com,
        xiang@kernel.org, chao@kernel.org,
        damien.lemoal@opensource.wdc.com, jth@kernel.org,
        linux-erofs@lists.ozlabs.org, linux-btrfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
        cluster-devel@redhat.com
Subject: Re: [PATCH v2 09/23] iomap: allow filesystem to implement read path
 verification
Message-ID: <20230405150627.GC303486@frogsfrogsfrogs>
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 <20230405110116.ia5wv3qxbnpdciui@aalbersh.remote.csb>
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On Wed, Apr 05, 2023 at 01:01:16PM +0200, Andrey Albershteyn wrote:
> Hi Christoph,
> 
> On Tue, Apr 04, 2023 at 08:37:02AM -0700, Christoph Hellwig wrote:
> > >  	if (iomap_block_needs_zeroing(iter, pos)) {
> > >  		folio_zero_range(folio, poff, plen);
> > > +		if (iomap->flags & IOMAP_F_READ_VERITY) {
> > 
> > Wju do we need the new flag vs just testing that folio_ops and
> > folio_ops->verify_folio is non-NULL?
> 
> Yes, it can be just test, haven't noticed that it's used only here,
> initially I used it in several places.
> 
> > 
> > > -		ctx->bio = bio_alloc(iomap->bdev, bio_max_segs(nr_vecs),
> > > -				     REQ_OP_READ, gfp);
> > > +		ctx->bio = bio_alloc_bioset(iomap->bdev, bio_max_segs(nr_vecs),
> > > +				REQ_OP_READ, GFP_NOFS, &iomap_read_ioend_bioset);
> > 
> > All other callers don't really need the larger bioset, so I'd avoid
> > the unconditional allocation here, but more on that later.
> 
> Ok, make sense.
> 
> > 
> > > +		ioend = container_of(ctx->bio, struct iomap_read_ioend,
> > > +				read_inline_bio);
> > > +		ioend->io_inode = iter->inode;
> > > +		if (ctx->ops && ctx->ops->prepare_ioend)
> > > +			ctx->ops->prepare_ioend(ioend);
> > > +
> > 
> > So what we're doing in writeback and direct I/O, is to:
> > 
> >  a) have a submit_bio hook
> >  b) allow the file system to then hook the bi_end_io caller
> >  c) (only in direct O/O for now) allow the file system to provide
> >     a bio_set to allocate from
> 
> I see.
> 
> > 
> > I wonder if that also makes sense and keep all the deferral in the
> > file system.  We'll need that for the btrfs iomap conversion anyway,
> > and it seems more flexible.  The ioend processing would then move into
> > XFS.
> > 
> 
> Not sure what you mean here.

I /think/ Christoph is talking about allowing callers of iomap pagecache
operations to supply a custom submit_bio function and a bio_set so that
filesystems can add in their own post-IO processing and appropriately
sized (read: minimum you can get away with) bios.  I imagine btrfs has
quite a lot of (read) ioend processing they need to do, as will xfs now
that you're adding fsverity.

> > > @@ -156,6 +160,11 @@ struct iomap_folio_ops {
> > >  	 * locked by the iomap code.
> > >  	 */
> > >  	bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
> > > +
> > > +	/*
> > > +	 * Verify folio when successfully read
> > > +	 */
> > > +	bool (*verify_folio)(struct folio *folio, loff_t pos, unsigned int len);

Any reason why we shouldn't return the usual negative errno?

> > Why isn't this in iomap_readpage_ops?
> > 
> 
> Yes, it can be. But it appears to me to be more relevant to
> _folio_ops, any particular reason to move it there? Don't mind
> moving it to iomap_readpage_ops.

I think the point is that this is a general "check what we just read"
hook, so it could be in readpage_ops since we're never going to need to
re-validate verity contents, right?  Hence it could be in readpage_ops
instead of the general iomap_folio_ops.

<shrug> Is there a use case for ->verify_folio that isn't a read post-
processing step?

--D

> -- 
> - Andrey
>