linux-kernel - Re: [PATCH 03/33] mm: Implement readahead

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Date:   Wed, 17 Feb 2021 22:49:18 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@....de>,
        Mike Marshall <hubcap@...ibond.com>,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/33] mm: Implement readahead_control pageset expansion

On Wed, Feb 17, 2021 at 10:34:39PM +0000, David Howells wrote:
> Matthew Wilcox <willy@...radead.org> wrote:
> 
> > We're defeating the ondemand_readahead() algorithm here.  Let's suppose
> > userspace is doing 64kB reads, the filesystem is OrangeFS which only
> > wants to do 4MB reads, the page cache is initially empty and there's
> > only one thread doing a sequential read.  ondemand_readahead() calls
> > get_init_ra_size() which tells it to allocate 128kB and set the async
> > marker at 64kB.  Then orangefs calls readahead_expand() to allocate the
> > remainder of the 4MB.  After the app has read the first 64kB, it comes
> > back to read the next 64kB, sees the readahead marker and tries to trigger
> > the next batch of readahead, but it's already present, so it does nothing
> > (see page_cache_ra_unbounded() for what happens with pages present).
> 
> It sounds like Christoph is right on the right track and the vm needs to ask
> the filesystem (and by extension, the cache) before doing the allocation and
> before setting the trigger flag.  Then we don't need to call back into the vm
> to expand the readahead.

Doesn't work.  You could read my reply to Christoph, or try to figure out
how to get rid of
https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n742
for yourself.

> Also, there's Steve's request to try and keep at least two requests in flight
> for CIFS/SMB at the same time to consider.

That's not relevant to this problem.