Re: [PATCH 1/3 v2] mm: Batch unmapping of file mapped pages inshrink_page_list

From: Mel Gorman
Date: Fri Sep 14 2012 - 04:56:37 EST

On Thu, Sep 13, 2012 at 09:06:10AM -0700, Tim Chen wrote:
> On Tue, 2012-09-11 at 12:05 +0100, Mel Gorman wrote:
> >
> > One *massive* change here that is not called out in the changelog is that
> > the reclaim path now holds the page lock on multiple pages at the same
> > time waiting for them to be batch unlocked in __remove_mapping_batch.
> > This is suspicious for two reasons.
> >
> > The first suspicion is that it is expected that there are filesystems
> > that lock multiple pages in page->index order and page reclaim tries to
> > lock pages in a random order. You are "ok" because you trylock the pages
> > but there should be a comment explaining the situation and why you're
> > ok.
> >
> > My *far* greater concern is that the hold time for a locked page is
> > now potentially much longer. You could lock a bunch of filesystem pages
> > and then call pageout() on an swapcache page that takes a long time to
> > write. This potentially causes a filesystem (or flusher threads etc)
> > to stall on lock_page and that could cause all sorts of latency trouble.
> > It will be hard to hit this bug and diagnose it but I believe it's
> > there.
> >
> > That second risk *really* must be commented upon and ideally reviewed by
> > the filesystem people. However, I very strongly suspect that the outcome
> > of such a review will be a suggestion to unlock the pages and reacquire
> > the lock in __remove_mapping_batch(). Bear in mind that if you take this
> > approach that you *must* use trylock when reacquiring the page lock and
> > handle being unable to lock the page.
> >
> Mel,
> Thanks for your detailed comments and analysis. If I unlock the pages,
> will flusher threads be the only things that will touch them?

I don't think so. The pages are still in the mappings radix tree so
potentially can be still read()/write() to and recreate empty buffers
potentially. Something like;

writepage handler
-> block_write_full_page
-> block_write_full_page_endio
-> __block_write_full_page
-> create_empty_buffers

Offhand I would also expect it's possible to fault the page again once
the page lock is released.

> Or do I
> have to worry about potentially other things done to the pages that will
> make it invalid for me to unmap the pages later and put them on free
> list?

I expect that you'll have to double check that the page is still eligible
to be removed from the mapping and added to the free list. This could get
complex because you then have to retry reclaim with those pages without
any batching and it may offset the advantage you are measuring.

You have potentially another option as well that you should consider. I
was complaining about holding multiple locks because of the potentially
unbounded length of time you hold those locks. It now occurs to me that
you could hold these locks but call __remove_mapping_batch() and drain
the list before calling long-lived operations like pageout(). That might
be easier to implement.

Mel Gorman
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