Re: [PATCH 23/28] mm, page_alloc: Check multiple page fields with a single branch

[Vlastimil Babka]

On 04/15/2016 11:07 AM, Mel Gorman wrote:
> Every page allocated or freed is checked for sanity to avoid corruptions
> that are difficult to detect later.  A bad page could be due to a number of
> fields. Instead of using multiple branches, this patch combines multiple
> fields into a single branch. A detailed check is only necessary if that
> check fails.
>
>                                             4.6.0-rc2                  4.6.0-rc2
>                                        initonce-v1r20            multcheck-v1r20
> Min      alloc-odr0-1               359.00 (  0.00%)           348.00 (  3.06%)
> Min      alloc-odr0-2               260.00 (  0.00%)           254.00 (  2.31%)
> Min      alloc-odr0-4               214.00 (  0.00%)           213.00 (  0.47%)
> Min      alloc-odr0-8               186.00 (  0.00%)           186.00 (  0.00%)
> Min      alloc-odr0-16              173.00 (  0.00%)           173.00 (  0.00%)
> Min      alloc-odr0-32              165.00 (  0.00%)           166.00 ( -0.61%)
> Min      alloc-odr0-64              162.00 (  0.00%)           162.00 (  0.00%)
> Min      alloc-odr0-128             161.00 (  0.00%)           160.00 (  0.62%)
> Min      alloc-odr0-256             170.00 (  0.00%)           169.00 (  0.59%)
> Min      alloc-odr0-512             181.00 (  0.00%)           180.00 (  0.55%)
> Min      alloc-odr0-1024            190.00 (  0.00%)           188.00 (  1.05%)
> Min      alloc-odr0-2048            196.00 (  0.00%)           194.00 (  1.02%)
> Min      alloc-odr0-4096            202.00 (  0.00%)           199.00 (  1.49%)
> Min      alloc-odr0-8192            205.00 (  0.00%)           202.00 (  1.46%)
> Min      alloc-odr0-16384           205.00 (  0.00%)           203.00 (  0.98%)
>
> Again, the benefit is marginal but avoiding excessive branches is
> important. Ideally the paths would not have to check these conditions at
> all but regrettably abandoning the tests would make use-after-free bugs
> much harder to detect.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

I wonder, would it be just too ugly to add +1 to atomic_read(&page->_mapcount) 
and OR it with the rest for a truly single branch?