linux-ext4 - Re: [RFCv3 7/7] iomap: Optimize data access patterns for filesystems with indirect mappings

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Date: Fri, 26 Apr 2024 20:19:47 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: "Darrick J. Wong" <djwong@...nel.org>, linux-ext4@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFCv3 7/7] iomap: Optimize data access patterns for filesystems
 with indirect mappings

On Sat, Apr 27, 2024 at 12:27:52AM +0530, Ritesh Harjani wrote:
> Matthew Wilcox <willy@...radead.org> writes:
> > @@ -79,6 +79,7 @@ static void iomap_set_range_uptodate(struct folio *folio, size_t off,
> >  	if (ifs) {
> >  		spin_lock_irqsave(&ifs->state_lock, flags);
> >  		uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
> > +		ifs->read_bytes_pending -= len;
> >  		spin_unlock_irqrestore(&ifs->state_lock, flags);
> >  	}
> 
> iomap_set_range_uptodate() gets called from ->write_begin() and
> ->write_end() too. So what we are saying is we are updating
> the state of read_bytes_pending even though we are not in
> ->read_folio() or ->readahead() call?

Exactly.

> >  
> > @@ -208,6 +209,8 @@ static struct iomap_folio_state *ifs_alloc(struct inode *inode,
> >  	spin_lock_init(&ifs->state_lock);
> >  	if (folio_test_uptodate(folio))
> >  		bitmap_set(ifs->state, 0, nr_blocks);
> > +	else
> > +		ifs->read_bytes_pending = folio_size(folio);
> 
> We might not come till here during ->read_folio -> ifs_alloc(). Since we
> might have a cached ifs which was allocated during write to this folio.
> 
> But unless you are saying that during writes, we would have set
> ifs->r_b_p to folio_size() and when the read call happens, we use
> the same value of the cached ifs.
> Ok, I see. I was mostly focusing on updating ifs->r_b_p value only when
> the reads bytes are actually pending during ->read_folio() or
> ->readahead() and not updating r_b_p during writes.

I see why you might want to think that way ... but this way is much less
complex, don't you think?  ;-)

> ...One small problem which I see with this approach is - we might have
> some non-zero value in ifs->r_b_p when ifs_free() gets called and it
> might give a warning of non-zero ifs->r_b_p, because we updated
> ifs->r_b_p during writes to a non-zero value, but the reads
> never happend. Then during a call to ->release_folio, it will complain
> of a non-zero ifs->r_b_p.

Yes, we'll have to remove that assertion.  I don't think that's a
problem, do you?