Re: [PATCH] thp: tail page refcounting fix #4

[Michel Lespinasse <walken@google.com>]

On Sat, Aug 27, 2011 at 10:34 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> Subject: thp: tail page refcounting fix
>
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> Michel while working on the working set estimation code, noticed that calling
> get_page_unless_zero() on a random pfn_to_page(random_pfn) wasn't safe, if the
> pfn ended up being a tail page of a transparent hugepage under splitting by
> __split_huge_page_refcount(). He then found the problem could also
> theoretically materialize with page_cache_get_speculative() during the
> speculative radix tree lookups that uses get_page_unless_zero() in SMP if the
> radix tree page is freed and reallocated and get_user_pages is called on it
> before page_cache_get_speculative has a chance to call get_page_unless_zero().
>
> So the best way to fix the problem is to keep page_tail->_count zero at all
> times. This will guarantee that get_page_unless_zero() can never succeed on any
> tail page. page_tail->_mapcount is guaranteed zero and is unused for all tail
> pages of a compound page, so we can simply account the tail page references
> there and transfer them to tail_page->_count in __split_huge_page_refcount() (in
> addition to the head_page->_mapcount).
>
> While debugging this s/_count/_mapcount/ change I also noticed get_page is
> called by direct-io.c on pages returned by get_user_pages. That wasn't entirely
> safe because the two atomic_inc in get_page weren't atomic. As opposed other
> get_user_page users like secondary-MMU page fault to establish the shadow
> pagetables would never call any superflous get_page after get_user_page
> returns. It's safer to make get_page universally safe for tail pages and to use
> get_page_foll() within follow_page (inside get_user_pages()). get_page_foll()
> is safe to do the refcounting for tail pages without taking any locks because
> it is run within PT lock protected critical sections (PT lock for pte and
> page_table_lock for pmd_trans_huge). The standard get_page() as invoked by
> direct-io instead will now take the compound_lock but still only for tail
> pages. The direct-io paths are usually I/O bound and the compound_lock is per
> THP so very finegrined, so there's no risk of scalability issues with it. A
> simple direct-io benchmarks with all lockdep prove locking and spinlock
> debugging infrastructure enabled shows identical performance and no overhead.
> So it's worth it. Ideally direct-io should stop calling get_page() on pages
> returned by get_user_pages(). The spinlock in get_page() is already optimized
> away for no-THP builds but doing get_page() on tail pages returned by GUP is
> generally a rare operation and usually only run in I/O paths.
>
> This new refcounting on page_tail->_mapcount in addition to avoiding new RCU
> critical sections will also allow the working set estimation code to work
> without any further complexity associated to the tail page refcounting
> with THP.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Reported-by: Michel Lespinasse <walken@google.com>
> Reviewed-by: Michel Lespinasse <walken@google.com>

Looks great.

I think some page_mapcount call sites would be easier to read if you
took on my tail_page_count() suggestion (so we can casually see it's a
refcount rather than mapcount). But you don't have to do it if you
don't think it helps. I'm happy enough with the code already :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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