Re: [PATCH] mm/huge_memory: prevent THP_ZERO_PAGE_ALLOC increased twice

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 6 Sep 2022 09:52:23 +0800 Liu Shixin <liushixin2@huawei.com> wrote:

> On 2022/9/6 4:07, Andrew Morton wrote:
> > On Mon, 5 Sep 2022 21:38:13 +0800 Liu Shixin <liushixin2@huawei.com> wrote:
> >
> >> If two or more threads call get_huge_zero_page concurrently, THP_ZERO_PAGE_ALLOC
> >> may increased two or more times. But actually, this should only count
> >> as once since the extra zero pages has been freed.
> > Well, for better of for worse,
> > Documentation/admin-guide/mm/transhuge.rst says
> >
> > thp_zero_page_alloc
> > 	is incremented every time a huge zero page is
> > 	successfully allocated. It includes allocations which where
> > 	dropped due race with other allocation. Note, it doesn't count
> > 	every map of the huge zero page, only its allocation.
> >
> > If you think this interprtation should be changed then please explain
> > why, and let's be sure to update the documentation accordingly.
> >
> > .
> Thanks for your explanation. I misunderstood the meaning of thp_zero_page_alloc before.
> Although the rules are clearly explained in the documentation, I think that this variable
> should only incremented when a huge zero page used for thp is successfully allocated and
> the pages dropped due race should skip increment. It seems strange to count in all allocations.
> 
> If there's something I still misunderstand, please point it out, thanks.

It seems strange to me also.  Perhaps there's a rationale buried in the
git and mailing list history.