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From: Matthew Wilcox <willy@infradead.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: Chris Mason <clm@fb.com>,
	"linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"jlayton@kernel.org" <jlayton@kernel.org>,
	"hch@infradead.org" <hch@infradead.org>
Subject: Re: [RFC 0/8] Replacing the readpages a_op
Date: Mon, 13 Jan 2020 14:27:04 -0800	[thread overview]
Message-ID: <20200113222704.GC18216@bombadil.infradead.org> (raw)
In-Reply-To: <1b94e6b6-29dc-2e90-d1ca-982accd3758c@kernel.dk>

On Mon, Jan 13, 2020 at 03:14:26PM -0700, Jens Axboe wrote:
> On 1/13/20 3:10 PM, Matthew Wilcox wrote:
> > On Mon, Jan 13, 2020 at 03:00:40PM -0700, Jens Axboe wrote:
> >> On 1/13/20 2:58 PM, Matthew Wilcox wrote:
> >>> On Mon, Jan 13, 2020 at 06:00:52PM +0000, Chris Mason wrote:
> >>>> This is true, I didn't explain that part well ;)  Depending on 
> >>>> compression etc we might end up poking the xarray inside the actual IO 
> >>>> functions, but the main difference is that btrfs is building a single 
> >>>> bio.  You're moving the plug so you'll merge into single bio, but I'd 
> >>>> rather build 2MB bios than merge them.
> >>>
> >>> Why don't we store a bio pointer inside the plug?  You're opencoding that,
> >>> iomap is opencoding that, and I bet there's a dozen other places where
> >>> we pass a bio around.  Then blk_finish_plug can submit the bio.
> >>
> >> Plugs aren't necessarily a bio, they can be callbacks too.
> > 
> > I'm thinking something as simple as this:
>
> It's a little odd imho, the plugging generally collect requests. Sounds
> what you're looking for is some plug owner private data, which just
> happens to be a bio in this case?
> 
> Is this over repeated calls to some IO generating helper? Would it be
> more efficient if that helper could generate the full bio in one go,
> instead of piecemeal?

The issue is around ->readpages.  Take a look at how iomap_readpages
works, for example.  We're under a plug (taken in mm/readahead.c),
but we still go through the rigamarole of keeping a pointer to the bio
in ctx.bio and passing ctx around so that we don't end up with many
fragments which have to be recombined into a single bio at the end.

I think what I want is a bio I can reach from current, somehow.  And the
plug feels like a natural place to keep it because it's basically saying
"I want to do lots of little IOs and have them combined".  The fact that
the iomap code has a bio that it precombines fragments into suggests to
me that the existing antifragmentation code in the plugging mechanism
isn't good enough.  So let's make it better by storing a bio in the plug
and then we can get rid of the bio in the iomap code.

  reply	other threads:[~2020-01-13 22:27 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-13 15:37 [RFC 0/8] Replacing the readpages a_op Matthew Wilcox
2020-01-13 15:37 ` [PATCH 1/8] pagevec: Add an iterator Matthew Wilcox
2020-01-13 15:37 ` [PATCH 2/8] mm: Fix the return type of __do_page_cache_readahead Matthew Wilcox
2020-01-13 15:37 ` [PATCH 3/8] mm: Use a pagevec for readahead Matthew Wilcox
2020-01-13 15:37 ` [PATCH 4/8] mm/fs: Add a_ops->readahead Matthew Wilcox
2020-01-13 18:22   ` Daniel Wagner
2020-01-13 19:17     ` Matthew Wilcox
2020-01-13 15:37 ` [PATCH 5/8] iomap,xfs: Convert from readpages to readahead Matthew Wilcox
2020-01-13 15:37 ` [PATCH 6/8] cifs: " Matthew Wilcox
2020-01-13 15:37 ` [PATCH 7/8] mm: Remove add_to_page_cache_locked Matthew Wilcox
2020-01-13 15:37 ` [PATCH 8/8] mm: Unify all add_to_page_cache variants Matthew Wilcox
2020-01-13 16:42 ` [RFC 0/8] Replacing the readpages a_op Chris Mason
2020-01-13 17:40   ` Matthew Wilcox
2020-01-13 18:00     ` Chris Mason
2020-01-13 21:58       ` Matthew Wilcox
2020-01-13 22:00         ` Jens Axboe
2020-01-13 22:10           ` Matthew Wilcox
2020-01-13 22:14             ` Jens Axboe
2020-01-13 22:27               ` Matthew Wilcox [this message]
2020-01-13 22:30                 ` Jens Axboe
2020-01-13 22:34                 ` Chris Mason
2020-01-14  1:01                   ` Matthew Wilcox
2020-01-14  1:07                     ` Chris Mason
2020-01-13 17:54   ` Matthew Wilcox
2020-01-13 22:19     ` Jens Axboe

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