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

[Vlastimil Babka]

On 05/15/2015 01:43 PM, Kirill A. Shutemov wrote:
> 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.

Ah, OK.

> > 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.

BUILD_BUG() seems like a better match here. Better than pollute the code 
for THP=n (in case of Patch 3 where it doesn't eliminate).

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

That's why I suggested a different variant of the "variable".
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/