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

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Date:   Tue, 16 Feb 2021 11:32:15 +0100
From:   Christoph Hellwig <hch@....de>
To:     David Howells <dhowells@...hat.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.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

On Mon, Feb 15, 2021 at 03:44:52PM +0000, David Howells wrote:
> Provide a function, readahead_expand(), that expands the set of pages
> specified by a readahead_control object to encompass a revised area with a
> proposed size and length.
> 
> The proposed area must include all of the old area and may be expanded yet
> more by this function so that the edges align on (transparent huge) page
> boundaries as allocated.
> 
> The expansion will be cut short if a page already exists in either of the
> areas being expanded into.  Note that any expansion made in such a case is
> not rolled back.
> 
> This will be used by fscache so that reads can be expanded to cache granule
> boundaries, thereby allowing whole granules to be stored in the cache, but
> there are other potential users also.

So looking at linux-next this seems to have a user, but that user is
dead wood given that nothing implements ->expand_readahead.

Looking at the code structure I think netfs_readahead and
netfs_rreq_expand is a complete mess and needs to be turned upside
down, that is instead of calling back from netfs_readahead to the
calling file system, split it into a few helpers called by the
caller.

But even after this can't we just expose the cache granule boundary
to the VM so that the read-ahead request gets setup correctly from
the very beginning?