linux-kernel - Re: [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge

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Message-Id: <20220816162024.60087b143995d9e21413fc52@linux-foundation.org>
Date:   Tue, 16 Aug 2022 16:20:24 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, songmuchun@...edance.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge_pages

On Tue, 16 Aug 2022 15:52:47 -0700 Mike Kravetz <mike.kravetz@...cle.com> wrote:

> On 08/16/22 21:05, Miaohe Lin wrote:
> > There should be pages_per_huge_page(h) / pages_per_huge_page(target_hstate)
> > pages incremented for target_hstate->max_huge_pages when page is demoted.
> > Update max_huge_pages accordingly for consistency.
> > 
> > Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> > ---
> >  mm/hugetlb.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index ea1c7bfa1cc3..e72052964fb5 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -3472,7 +3472,8 @@ static int demote_free_huge_page(struct hstate *h, struct page *page)
> >  	 * based on pool changes for the demoted page.
> >  	 */
> >  	h->max_huge_pages--;
> > -	target_hstate->max_huge_pages += pages_per_huge_page(h);
> > +	target_hstate->max_huge_pages +=
> > +		pages_per_huge_page(h) / pages_per_huge_page(target_hstate);
> 
> Thanks!
> 
> That is indeed incorrect.  However the miscalculation should not have any 
> consequences.  Correct?  The value is used when initially populating the
> pools.  It is never read and used again.  It is written to in
> set_max_huge_pages if someone changes the number of hugetlb pages.
> 
> I guess that is a long way of saying I am not sure why we care about trying
> to keep max_huge_pages up to date?  I do not think it matters.
> 
> I also thought, if we are going to adjust max_huge_pages here we may
> also want to adjust the node specific value: h->max_huge_pages_node[node].
> There are a few other places where the global max_huge_pages is adjusted
> without adjusting the node specific value.
> 
> The more I think about it, the more I think we should explore just
> eliminating any adjustment of this/these values after initially
> populating the pools.

I'm thinking we should fix something that is "indeed incorrect" before
going on to more extensive things?