Re: [PATCH] mm: avoid tail page refcounting on non-THP compound pages

[Kirill A. Shutemov]

On Wed, Mar 25, 2015 at 03:48:48PM -0700, Hugh Dickins wrote:
> On Wed, 25 Mar 2015, Hugh Dickins wrote:
> > On Thu, 26 Mar 2015, Kirill A. Shutemov wrote:
> > 
> > > THP uses tail page refcounting to be able to split huge page at any
> > > time. Tail page refcounting is not needed for rest users of compound
> > > pages and it's harmful because of overhead.
> > > 
> > > We try to exclude non-THP pages from tail page refcounting using
> > > __compound_tail_refcounted() check. It excludes most common non-THP
> > > compound pages: SL*B and hugetlb, but it doesn't catch rest of
> > > __GFP_COMP users -- drivers.
> > > 
> > > And it's not only about overhead.
> > > 
> > > Drivers might want to use compound pages to get refcounting semantics
> > > suitable for mapping high-order pages to userspace. But tail page
> > > refcounting breaks it.
> > > 
> > > Tail page refcounting uses ->_mapcount in tail pages to store GUP pins
> > > on them. It means GUP pins would affect page_mapcount() for tail pages.
> > > It's not a problem for THP, because it never maps tail pages. But unlike
> > > THP, drivers map parts of compound pages with PTEs and it makes
> > > page_mapcount() be called for tail pages.
> > > 
> > > In particular, GUP pins would shift PSS up and affect /proc/kpagecount
> > > for such pages. But, I'm not aware about anything which can lead to
> > > crash or other serious misbehaviour.
> > > 
> > > Since currently all THP pages are anonymous and all drivers pages are
> > > not, we can fix the __compound_tail_refcounted() check by requiring
> > > PageAnon() to enable tail page refcounting.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > 
> > Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> 
> Oh, hold on a moment: does this actually build in a tree without your
> page-flags.h consolidation?  It didn't when I tried to add a PageAnon
> test there for my series against v3.19, has something changed in v4.0?

No. I haven't tried to build it without my patchset, but it seems it
wouldn't.

Just check: it would build for me on top of [PATCH 01/16], you've acked.

-- 
 Kirill A. Shutemov
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