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

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Message-ID: <2083368.1613670420@warthog.procyon.org.uk>
Date:   Thu, 18 Feb 2021 17:47:00 +0000
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.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>,
        Christoph Hellwig <hch@....de>, 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

Matthew Wilcox <willy@...radead.org> wrote:

> So readahead_expand() needs to adjust the file's f_ra so that when the
> application gets to 64kB, it kicks off the readahead of 4MB-8MB chunk (and
> then when we get to 4MB+256kB, it kicks off the readahead of 8MB-12MB,
> and so on).

Ummm...  Two questions:

Firstly, how do I do that?  Set ->async_size?  And to what?  The expansion
could be 2MB from a ceph stripe, 256k from the cache.  Just to add to the fun,
the leading edge of the window might also be rounded downwards and the RA
trigger could be before where the app is going to start reading.

Secondly, what happens if, say, a 4MB read is covered by a single 4MB THP?

David