Re: [PATCH 10/10] mm/hugetlb: not necessary to abuse temporary page to workaround the nasty free_huge_page

Baoquan He
2020-08-11T01:51:59Z

On 08/10/20 at 05:19pm, Mike Kravetz wrote:
> On 8/9/20 7:17 PM, Baoquan He wrote:
> > On 08/07/20 at 05:12pm, Wei Yang wrote:
> >> Let's always increase surplus_huge_pages and so that free_huge_page
> >> could decrease it at free time.
> >>
> >> Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
> >> ---
> >> mm/hugetlb.c | 14 ++++++--------
> >> 1 file changed, 6 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index 1f2010c9dd8d..a0eb81e0e4c5 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -1913,21 +1913,19 @@ static struct page *alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
> >> return NULL;
> >>
> >> spin_lock(&hugetlb_lock);
> >> +
> >> + h->surplus_huge_pages++;
> >> + h->surplus_huge_pages_node[page_to_nid(page)]++;
> >> +
> >> /*
> >> * We could have raced with the pool size change.
> >> * Double check that and simply deallocate the new page
> >> - * if we would end up overcommiting the surpluses. Abuse
> >> - * temporary page to workaround the nasty free_huge_page
> >> - * codeflow
> >> + * if we would end up overcommiting the surpluses.
> >> */
> >> - if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
> >> - SetPageHugeTemporary(page);
> >
> > Hmm, the temporary page way is taken intentionally in
> > commit 9980d744a0428 ("mm, hugetlb: get rid of surplus page accounting tricks").
> > From code, this is done inside hugetlb_lock holding, and the code flow
> > is straightforward, should be safe. Adding Michal to CC.
> >
>
> I remember when the temporary page code was added for page migration.
> The use of temporary page here was added at about the same time. Temporary
> page does have one advantage in that it will not CAUSE surplus count to
> exceed overcommit. This patch could cause surplus to exceed overcommit
> for a very short period of time. However, do note that for this to happen
> the code needs to race with a pool resize which itself could cause surplus
> to exceed overcommit.
>
> IMO both approaches are valid.
> - Advantage of temporary page is that it can not cause surplus to exceed
> overcommit. Disadvantage is as mentioned in the comment 'abuse of temporary
> page'.
> - Advantage of this patch is that it uses existing counters. Disadvantage
> is that it can momentarily cause surplus to exceed overcommit.

Yeah, since it's all done inside hugetlb_lock, should be OK even
though it may cause surplus to exceed overcommit.
>
> Unless someone has a strong opinion, I prefer the changes in this patch.

Agree, I also prefer the code change in this patch, to remove the
unnecessary confusion about the temporary page.