Re: [PATCH 03/33] mm: Implement readahead_control pageset expansion

[David Howells]

Christoph Hellwig <hch@xxxxxx> wrote:

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

Interesting question.  Code on my fscache-iter branch does implement this, but
I was asked to split the patchset up, so that's not in this subset.

> 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?

You need to argue this one with Willy.  In my opinion, the VM should ask the
filesystem and the expansion be done before ->readahead() is called.  Willy
disagrees, however.

David