linux-ext4 - Re: [PATCH v2 09/23] iomap: allow filesystem to implement read path verification

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Message-ID: <20230405110116.ia5wv3qxbnpdciui@aalbersh.remote.csb>
Date:   Wed, 5 Apr 2023 13:01:16 +0200
From:   Andrey Albershteyn <aalbersh@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     djwong@...nel.org, dchinner@...hat.com, ebiggers@...nel.org,
        linux-xfs@...r.kernel.org, fsverity@...ts.linux.dev,
        rpeterso@...hat.com, agruenba@...hat.com, xiang@...nel.org,
        chao@...nel.org, damien.lemoal@...nsource.wdc.com, jth@...nel.org,
        linux-erofs@...ts.ozlabs.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com
Subject: Re: [PATCH v2 09/23] iomap: allow filesystem to implement read path
 verification

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.

> > @@ -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);
> 
> 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.

-- 
- Andrey