Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig_range handle in-use hugetlb pages

[David Hildenbrand]

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@xxxxxxx>
> > > ---
> > >    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 :) )

--
Thanks,

David / dhildenb