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Date:   Wed, 17 Feb 2021 22:49:18 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Mike Marshall <hubcap@omnibond.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Steve French <sfrench@samba.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, linux-mm@kvack.org,
        linux-cachefs@redhat.com, linux-afs@lists.infradead.org,
        linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org,
        ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org, Jeff Layton <jlayton@redhat.com>,
        David Wysochanski <dwysocha@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 03/33] mm: Implement readahead_control pageset expansion
Message-ID: <20210217224918.GP2858050@casper.infradead.org>
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 <1901187.1613601279@warthog.procyon.org.uk>
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On Wed, Feb 17, 2021 at 10:34:39PM +0000, David Howells wrote:
> Matthew Wilcox <willy@infradead.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.