Re: [PATCH 02/21] mm/hugetlb: correct max_huge_pages accounting on demote

[Mike Kravetz]

On 09/14/22 13:58, Andrew Morton wrote:
> On Wed, 14 Sep 2022 10:23:05 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
> > On 09/13/22 12:54, Doug Berger wrote:
> > > When demoting a hugepage to a smaller order, the number of pages
> > > added to the target hstate will be the size of the large page
> > > divided by the size of the smaller page.
> > > 
> > > Fixes: 8531fc6f52f5 ("hugetlb: add hugetlb demote page support")
> > > Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
> > > ---
> > >  mm/hugetlb.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index e070b8593b37..79949893ac12 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);
> > >  
> > >  	return rc;
> > >  }
> > 
> > This has already been fixed here,
> > 
> > https://lore.kernel.org/linux-mm/20220823030209.57434-2-linmiaohe@xxxxxxxxxx/
> 
> Neither version tells us the user-visible runtime effects of the change :(

Sorry, I should have pushed harder on this with Miaohe's patch.

There are no user-visible runtime effects.  In fact, this change really causes
no functional change (unless I am mistaken and Miaohe did not correct me).
max_huge_pages is not used again until it is reset.  See my explanation at:
https://lore.kernel.org/linux-mm/YvwfvxXewnZpHQcz@monkey/
-- 
Mike Kravetz