Re: [PATCH] mm: fix hang on anon_vma->root->lock

[Hugh Dickins <hughd@google.com>]

On Fri, Aug 27, 2010 at 4:06 PM, Christoph Lameter <cl@linux.com> wrote:
> On Fri, 27 Aug 2010, Hugh Dickins wrote:
>
>> >> I do not see a second check (*after* taking the lock) in the patch
>>
>>         if (page_mapped(page))
>>                 return anon_vma;
>
> As far as I can tell you would have to recheck the mapping pointer and the

That's a more interesting question than I'd realized.  When
page_lock_anon_vma() first came in (2.6.9) there was nothing which
updated page->mapping of an anon page after it was set, until the page
was freed.

Since then we've gathered a few places which update it while holding
the page lock (migrate.c, ksm.c) - no problem since the callers of
page_lock_anon_vma() hold and must hold page lock.  Well, there is the
fairly recent call to page_lock_anon_vma() from memory-failure.c, and
its even more recent use on hugepages: there's switching back and
forth between p and hpage and page, but I think it does end up
applying page_lock_anon_vma() to the very page that it locked earlier.

Then there's the recently added page_move_anon_rmap(): fine in
memory.c, the page lock is held; but apparently broken in hugetlb.c,
where it's called only when the pagelock has not been taken!
Horiguchi-san Cc'ed.

__page_set_anon_rmap() looks like it might have changed anon
page->mapping in 2.6.35, but Andrea has fixed that with PageAnon tests
in 2.6.36-rc.  Ah, but what if "exclusive" and non-exclusive calls to
__page_set_anon_rmap() are racing?  Not clear, it may be that Andrea
has only narrowed a window not closed it (and I've not yet looked up
the commit to see his intent); or it may be okay, that there cannot be
a conflict of anon_vma in that case.  Need to dig deeper.

__hugepage_set_anon_rmap() appears to copy the 2.6.35
__page_set_anon_rmap(), and probably needs to add in Andrea's fix, or
whatever else is needed there.

This is a different problem (or it may turn out to be a non-existent
problem, aside from the hugetlb.c case to be fixed there) than I was
fixing with my patch, and can be patched separately; but It certainly
looks as if it's worth adding a BUG_ON or VM_BUG_ON to check for a
switch of anon_vma beneath us there.   Plus a
VM_BUG_ON(PageLocked(page)) going into page_lock_anon_vma().

> pointer to the root too after taking the lock because only taking the lock
> stabilitzes the object.

A change in the pointer to the root is covered by the ACCESS_ONCE:
yes, it can change beneath us there, but only through the anon_vma
being freed and reused, in which case the subsequent page_mapped test
tells us the page is no longer mapped, whereupon we back out,
unlocking what we locked.  (I had at one point been tempted to check
anon_vma->root == root_anon_vma there instead of page_mapped(), but
that would not have been good enough: since anon_vma_prepare() sets
anon_vma->root before taking the lock, anon_vma->root could change
under us anywhere between the page_lock_anon_vma() and its
page_unlock_anon_vma() in that case.)

> Any other data you may have obtained before
> acquiring the lock may have changed.
>
>> >> and the way the lock is taken can be a problem in itself.
>>
>> No, that's what we rely upon SLAB_DESTROY_BY_RCU for.
>
> SLAB_DESTROY_BY_RCU does not guarantee that the object stays the same nor
> does it prevent any fields from changing. Going through a pointer with
> only SLAB_DESTROY_BY_RCU means that you can only rely on the atomicity
> guarantee for pointer updates. You get a valid pointer but pointer changes
> are not prevented by SLAB_DESTROY_BY_RCU.

You're speaking too generally there for me to understand its
relevance!  What specific problem do you see?

>
> The only guarantee of that would be through other synchronization
> techniques. If you believe that the page lock provides sufficient
> synchronization that then this approach may be ok.

The page lock should be guaranteeing that page->mapping (anon_vma)
cannot change underneath us; but there is some doubt on that above,
I'll report back when I've had enough quiet time to think through the
__set_page_anon_rmap() possibilities:
thanks for uncovering those doubts.

Hugh

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