Redesigning and modernising fscache

[Matthew Wilcox]

On Thu, Jan 14, 2021 at 10:45:06AM +0000, David Howells wrote:
> However, there've been some objections to the approach I've taken to
> implementing this.  The way I've done it is to disable the use of fscache by
> the five network filesystems that use it, remove much of the old code, put in
> the reimplementation, then cut the filesystems over.  I.e. rip-and-replace.
> It leaves unported filesystems unable to use it - but three of the five are
> done (afs, ceph, nfs), and I've supplied partially-done patches for the other
> two (9p, cifs).
> 
> It's been suggested that it's too hard to review this way and that either I
> should go for a gradual phasing in or build the new one in parallel.  The
> first is difficult because I want to change how almost everything in there
> works - but the parts are tied together; the second is difficult because there
> are areas that would *have* to overlap (the UAPI device file, the cache
> storage, the cache size limits and at least some state for managing these), so
> there would have to be interaction between the two variants.  One refinement
> of the latter would be to make the two implementations mutually exclusive: you
> can build one or the other, but not both.

My reservation with "build fscache2" is that it's going to take some
time to do, and I really want rid of ->readpages as soon as possible.

What I'd like to see is netfs_readahead() existing as soon as possible,
built on top of the current core.  Then filesystems can implement
netfs_read_request_ops one by one, and they become insulated from the
transition.