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

[Matthew Wilcox]

On Wed, Feb 17, 2021 at 10:34:39PM +0000, David Howells wrote:
> Matthew Wilcox <willy at 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.