Re: [PATCH] mm: thp: Acquire the anon_vma rwsem for lock during split

[Mel Gorman <mgorman@suse.de>]

On Sat, Jan 05, 2013 at 06:24:47AM -0600, Simon Jeons wrote:
> On Fri, 2013-01-04 at 17:32 -0800, Michel Lespinasse wrote:
> > On Fri, Jan 4, 2013 at 6:08 AM, Mel Gorman <mgorman@suse.de> wrote:
> > > Despite the reason for these commits, NUMA balancing is not the direct
> > > source of the problem. split_huge_page() expected the anon_vma lock to be
> > > exclusive to serialise the whole split operation. Ordinarily it is expected
> > > that the anon_vma lock would only be required when updating the avcs but
> > > THP also uses it. The locking requirements for THP are complex and there
> > > is some overlap but broadly speaking they include the following
> > >
> > > 1. mmap_sem for read or write prevents THPs being created underneath
> > > 2. anon_vma is taken for write if collapsing a huge page
> > > 3. mm->page_table_lock should be taken when checking if pmd_trans_huge as
> > >    split_huge_page can run in parallel
> > > 4. wait_split_huge_page uses anon_vma taken for write mode to serialise
> > >    against other THP operations
> > > 5. compound_lock is used to serialise between
> > >    __split_huge_page_refcount() and gup
> > >
> > > split_huge_page takes anon_vma for read but that does not serialise against
> > > parallel split_huge_page operations on the same page (rule 2). One process
> > > could be modifying the ref counts while the other modifies the page tables
> > > leading to counters not being reliable. This patch takes the anon_vma
> > > lock for write to serialise against parallel split_huge_page and parallel
> > > collapse operations as it is the most fine-grained lock available that
> > > protects against both.
> > 
> > Your comment about this being the most fine-grained lock made me
> > think, couldn't we use lock_page() on the THP page here ?
> > 
> > Now I don't necessarily want to push you that direction, because I
> > haven't fully thought it trough and because what you propose brings us
> > closer to what happened before anon_vma became an rwlock, which is
> > more obviously safe. But I felt I should still mention it, since we're
> > really only trying to protect from concurrent operations on the same
> > THP page, so locking at just that granularity would seem desirable.
> 
> Why you said that anon_vma lock who will protect page associated to a
> list of vmas is fine-grained then page lock who just protect one page?
> 

We did not say it was fine-grained, we said it was the most fine-grained
lock available. It's a coarse lock but using the page lock would be
problematic.

-- 
Mel Gorman
SUSE Labs

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