Re: [PATCHv5 03/28] memcg: adjust to support new THP refcounting

On Fri, May 15, 2015 at 09:44:17AM +0200, Vlastimil Babka wrote:

On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:

quoted

As with rmap, with new refcounting we cannot rely on PageTransHuge() to
check if we need to charge size of huge page form the cgroup. We need to
get information from caller to know whether it was mapped with PMD or
PTE.

We do uncharge when last reference on the page gone. At that point if we
see PageTransHuge() it means we need to unchange whole huge page.

The tricky part is partial unmap -- when we try to unmap part of huge
page. We don't do a special handing of this situation, meaning we don't
uncharge the part of huge page unless last user is gone or
split_huge_page() is triggered. In case of cgroup memory pressure
happens the partial unmapped page will be split through shrinker. This
should be good enough.

Signed-off-by: Kirill A. Shutemov <redacted>
Tested-by: Sasha Levin <redacted>

Acked-by: Vlastimil Babka <redacted>

But same question about whether it should be using hpage_nr_pages() instead
of a constant.

No. Compiler woundn't be able to optimize HPAGE_PMD_NR away for THP=n,
since compound value cross compilation unit barrier.

-- 
 Kirill A. Shutemov

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