Re: [PATCH 23/43] CacheFiles: Permit the page lock state to be monitored [ver #46]

[David Howells <dhowells@redhat.com>]

Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Presumably: at the point where data is needed.

But the point where the data is needed is where filemap.c is waiting on a
netfs page.  Maybe the sync_page() aop can deal with it

There's also the problem of recording and pinning the backing page I'm waiting
for.  Currently I can do that by hooking the monitor block into the page
unlock watching list.  If I don't do that, I have to use up yet more memory to
track those some other way.  It's not impossible, but I'd like to keep memory
usage down.

> Or do you actually have numbers showing a problem if you just read the pages
> then copy them?

I did, years ago.  It wasn't particularly good, but
fscache_read_or_alloc_pages() was completely synchronous.

> If there is a problem, then why doesn't fscache_read_or_alloc_pages caller
> do the work itself, then you get as many threads as you have indivisible
> work units, so completing some part of the request before another wouldn't
> gain you anything anyway...

(1) Trond stipulated FS-Cache had to be asynchronous, and it is, as far as I
    can make it.  I still have to invoke bmap() synchronously, though, to find
    out whether I have a page in the cache to read:-/

(2) You lose the advantage of being able to process what you've got whilst the
    disk is fetching stuff in the background.

David