Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache

[Kirill A. Shutemov]

On Mon, Feb 17, 2014 at 11:01:58AM -0800, Linus Torvalds wrote:
> On Mon, Feb 17, 2014 at 10:38 AM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > Now we have ->fault_nonblock() to ask filesystem for a page, if it's
> > reachable without blocking. We request one page a time. It's not terribly
> > efficient and I will probably re-think the interface once again to expose
> > iterator or something...
> 
> Hmm. Yeah, clearly this isn't working, since the real workloads all
> end up looking like
> 
> >        115,493,976      minor-faults                                                  ( +-  0.00% ) [100.00%]
> >       59.686645587 seconds time elapsed                                          ( +-  0.30% )
>  becomes
> >         47,428,068      minor-faults                                                  ( +-  0.00% ) [100.00%]
> >       60.241766430 seconds time elapsed                                          ( +-  0.85% )
> 
> and
> 
> >        268,039,365      minor-faults                                                 [100.00%]
> >      132.830612471 seconds time elapsed
> becomes
> >        193,550,437      minor-faults                                                 [100.00%]
> >      132.851823758 seconds time elapsed
> 
> and
> 
> >          4,967,540      minor-faults                                                  ( +-  0.06% ) [100.00%]
> >       27.215434226 seconds time elapsed                                          ( +-  0.18% )
> becomes
> >          2,285,563      minor-faults                                                  ( +-  0.26% ) [100.00%]
> >       27.292854546 seconds time elapsed                                          ( +-  0.29% )
> 
> ie it shows a clear reduction in faults, but the added costs clearly
> eat up any wins and it all becomes (just _slightly_) slower.
> 
> Sad.
> 
> I do wonder if we really need to lock the pages we fault in. We lock
> them in order to test for being up-to-date and still mapped. The
> up-to-date check we don't really need to worry about: that we can test
> without locking by just reading "page->flags" atomically and verifying
> that it's uptodate and not locked.
> 
> The other reason to lock the page is:
> 
>  - for anonymous pages we need the lock for rmap, so the VM generally
> always locks the page. But that's not an issue for file-backed pages:
> the "rmap" for a filebacked page is just the page mapcount and the
> cgroup statistics, and those don't need the page lock.
> 
>  - the whole truncation/unmapping thing
> 
> So the complex part is racing with truncate/unmapping the page. But
> since we hold the page table lock, I *think* what we should be able to
> do is:
> 
>  - increment the page _mapcount (iow, do "page_add_file_rmap()"
> early). This guarantees that any *subsequent* unmap activity on this
> page will walk the file mapping lists, and become serialized by the
> page table lock we hold.
> 
>  - mb_after_atomic_inc() (this is generally free)
> 
>  - test that the page is still unlocked and uptodate, and the page
> mapping still points to our page.
> 
>  - if that is true, we're all good, we can use the page, otherwise we
> decrement the mapcount (page_remove_rmap()) and skip the page.
> 
> Hmm? Doing something like this means that we would never lock the
> pages we prefault, and you can go back to your gang lookup rather than
> that "one page at a time". And the race case is basically never going
> to trigger.
> 
> Comments?

Sounds reasonable to me. I'll take a closer look tomorrow.

But it could be safer to keep locking in place and reduce lookup cost by
exposing something like ->fault_iter_init() and ->fault_iter_next(). It
will still return one page a time, but it will keep radix-tree context
around for cheaper next-page lookup.

-- 
 Kirill A. Shutemov
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