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

[David Hildenbrand]

On 10.02.21 15:14, Oscar Salvador wrote:
> 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@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 :) )
>
> 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 :-)

Gigantic pages are a minefield. Not your fault :)

--
Thanks,

David / dhildenb