linux-kernel - Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig

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Date:   Wed, 10 Feb 2021 15:14:25 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig_range handle
 in-use hugetlb pages

On Wed, Feb 10, 2021 at 03:11:05PM +0100, David Hildenbrand wrote:
> On 10.02.21 15:09, Oscar Salvador wrote:
> > On Wed, Feb 10, 2021 at 09:56:37AM +0100, David Hildenbrand wrote:
> > > On 08.02.21 11:38, Oscar Salvador wrote:
> > > > alloc_contig_range is not prepared to handle hugetlb pages and will
> > > > fail if it ever sees one, but since they can be migrated as any other
> > > > page (LRU and Movable), it makes sense to also handle them.
> > > > 
> > > > For now, do it only when coming from alloc_contig_range.
> > > > 
> > > > Signed-off-by: Oscar Salvador <osalvador@...e.de>
> > > > ---
> > > >    mm/compaction.c | 17 +++++++++++++++++
> > > >    mm/vmscan.c     |  5 +++--
> > > >    2 files changed, 20 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > > index e5acb9714436..89cd2e60da29 100644
> > > > --- a/mm/compaction.c
> > > > +++ b/mm/compaction.c
> > > > @@ -940,6 +940,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> > > >    			goto isolate_fail;
> > > >    		}
> > > > +		/*
> > > > +		 * Handle hugetlb pages only when coming from alloc_contig
> > > > +		 */
> > > > +		if (PageHuge(page) && cc->alloc_contig) {
> > > > +			if (page_count(page)) {
> > > 
> > > I wonder if we should care about races here. What if someone concurrently
> > > allocates/frees?
> > > 
> > > Note that PageHuge() succeeds on tail pages, isolate_huge_page() not, i
> > > assume we'll have to handle that as well.
> > > 
> > > I wonder if it would make sense to move some of the magic to hugetlb code
> > > and handle it there with less chances for races (isolate if used,
> > > alloc-and-dissolve if not).
> > 
> > Yes, it makes sense to keep the magic in hugetlb code.
> > Note, though, that removing all races might be tricky.
> > 
> > isolate_huge_page() checks for PageHuge under hugetlb_lock,
> > so there is a race between a call to PageHuge(x) and a subsequent
> > call to isolate_huge_page().
> > But we should be fine as isolate_huge_page will fail in case the page is
> > no longer HugeTLB.
> > 
> > Also, since isolate_migratepages_block() gets called with ranges
> > pageblock aligned, we should never be handling tail pages in the core
> > of the function. E.g: the same way we handle THP:
> 
> Gigantic pages? (spoiler: see my comments to next patch :) )

Oh, yeah, that sucks.
We had the same problem in scan_movable_pages/has_unmovable_pages
with such pages.

Uhm, I will try to be more careful :-)

-- 
Oscar Salvador
SUSE L3