Re: [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge_pages

From: Mike Kravetz <mike.kravetz@oracle.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>,
	songmuchun@bytedance.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/6] mm/hugetlb: fix incorrect update of max_huge_pages
Date: Tue, 16 Aug 2022 16:34:47 -0700	[thread overview]
Message-ID: <Yvwpl/RD9tLr6HJE@monkey> (raw)
In-Reply-To: <20220816162024.60087b143995d9e21413fc52@linux-foundation.org>

On 08/16/22 16:20, Andrew Morton wrote:
> On Tue, 16 Aug 2022 15:52:47 -0700 Mike Kravetz <mike.kravetz@oracle.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@huawei.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?

Sure, I am good with that.

Just wanted to point out that the incorrect calculation does not have
any negative consequences.  Maybe prompting Miaohe to look into the more
extensive cleanup.
-- 
Mike Kravetz