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

[Simon Jeons]

On Fri, 2013-01-04 at 17:32 -0800, Michel Lespinasse wrote:
> On Fri, Jan 4, 2013 at 6:08 AM, Mel Gorman <mgorman@xxxxxxx> 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?

> 


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