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

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