linux-kernel - Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

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Message-ID: <20210112112709.GO22493@dhcp22.suse.cz>
Date:   Tue, 12 Jan 2021 12:27:09 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Muchun Song <songmuchun@...edance.com>, mike.kravetz@...cle.com,
        akpm@...ux-foundation.org, n-horiguchi@...jp.nec.com,
        ak@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose
 refcount is one

On Tue 12-01-21 12:11:21, David Hildenbrand wrote:
> On 12.01.21 12:00, David Hildenbrand wrote:
> > On 10.01.21 13:40, Muchun Song wrote:
> >> If the refcount is one when it is migrated, it means that the page
> >> was freed from under us. So we are done and do not need to migrate.
> >>
> >> This optimization is consistent with the regular pages, just like
> >> unmap_and_move() does.
> >>
> >> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> >> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
> >> Acked-by: Yang Shi <shy828301@...il.com>
> >> ---
> >>  mm/migrate.c | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/mm/migrate.c b/mm/migrate.c
> >> index 4385f2fb5d18..a6631c4eb6a6 100644
> >> --- a/mm/migrate.c
> >> +++ b/mm/migrate.c
> >> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
> >>  		return -ENOSYS;
> >>  	}
> >>  
> >> +	if (page_count(hpage) == 1) {
> >> +		/* page was freed from under us. So we are done. */
> >> +		putback_active_hugepage(hpage);
> >> +		return MIGRATEPAGE_SUCCESS;
> >> +	}
> >> +
> >>  	new_hpage = get_new_page(hpage, private);
> >>  	if (!new_hpage)
> >>  		return -ENOMEM;
> >>
> > 
> > Question: What if called via alloc_contig_range() where we even want to
> > "migrate" free pages, meaning, relocate it?
> > 
> 
> To be more precise:
> 
> a) We don't have dissolve_free_huge_pages() calls on the
> alloc_contig_range() path. So we *need* migration IIUC.
> 
> b) dissolve_free_huge_pages() will fail if going below the reservation.
> In that case we really want to migrate free pages. This even applies to
> memory offlining.
> 
> Either I am missing something important or this patch is more dangerous
> than it looks like.

This is an interesting point. But do we try to migrate hugetlb pages in
alloc_contig_range? isolate_migratepages_block !PageLRU need to be
marked as PageMovable AFAICS. This would be quite easy to implement but
a more fundamental question is whether we really want to mess with
existing pools for alloc_contig_range.

Anyway you are quite right that this change has more side effects than
it is easy to see while it doesn't really bring any major advantage
other than the code consistency.
-- 
Michal Hocko
SUSE Labs