Re: [PATCHv5 01/28] mm, proc: adjust PSS calculation

[Kirill A. Shutemov]

On Fri, May 15, 2015 at 01:33:31PM +0200, Vlastimil Babka wrote:
> On 05/15/2015 12:56 PM, Kirill A. Shutemov wrote:
> >On Thu, May 14, 2015 at 04:12:29PM +0200, Vlastimil Babka wrote:
> >>On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> >>>With new refcounting all subpages of the compound page are not nessessary
> >>>have the same mapcount. We need to take into account mapcount of every
> >>>sub-page.
> >>>
> >>>Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >>>Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> >>
> >>Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> >>
> >>(some nitpicks below)
> >>
> >>>---
> >>>  fs/proc/task_mmu.c | 43 ++++++++++++++++++++++---------------------
> >>>  1 file changed, 22 insertions(+), 21 deletions(-)
> >>>
> >>>diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> >>>index 956b75d61809..95bc384ee3f7 100644
> >>>--- a/fs/proc/task_mmu.c
> >>>+++ b/fs/proc/task_mmu.c
> >>>@@ -449,9 +449,10 @@ struct mem_size_stats {
> >>>  };
> >>>
> >>>  static void smaps_account(struct mem_size_stats *mss, struct page *page,
> >>>-		unsigned long size, bool young, bool dirty)
> >>>+		bool compound, bool young, bool dirty)
> >>>  {
> >>>-	int mapcount;
> >>>+	int i, nr = compound ? hpage_nr_pages(page) : 1;
> >>
> >>Why not just HPAGE_PMD_NR instead of hpage_nr_pages(page)?
> >
> >Okay, makes sense. Compiler is smart enough to optimize away HPAGE_PMD_NR
> >for THP=n. (HPAGE_PMD_NR is BUILD_BUG() for THP=n)
> 
> Ah, BUILD_BUG()... I'm not sure we can rely on optimization to avoid
> BUILD_BUG(), what if somebody compiles with all optimizations off?

Kernel relies on dead-code elimination. You cannot build kernel with -O0.

> So why not replace BUILD_BUG() with "1", or create a variant of HPAGE_PMD_NR
> that does that, for this case and patch 3. Seems better than testing
> PageTransHuge everywhere...

I think we could try to downgrade it BUG(). Although I found BUILD_BUG()
useful few times.

HPAGE_PMD_NR==1 would be just wrong. It would mean you can map order-0
page with PMD %-|

-- 
 Kirill A. Shutemov
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