linux-kernel - Re: [PATCH 10/10] mm/hugetlb: not necessary to abuse temporary page to workaround the nasty free_huge

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Date:   Tue, 11 Aug 2020 09:51:48 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>, mhocko@...nel.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] mm/hugetlb: not necessary to abuse temporary page
 to workaround the nasty free_huge_page

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@...ux.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.