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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Matthew Wilcox <willy@infradead.org> writes:

> On Sat, Apr 27, 2024 at 12:27:52AM +0530, Ritesh Harjani wrote:
>> Matthew Wilcox <willy@infradead.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?

Sure, I will give it a spin.

-ritesh