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

[David Hildenbrand]

On 11.02.21 01:56, Mike Kravetz wrote:
> On 2/8/21 2:38 AM, 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)) {
>
> Thanks for doing this!
>
> I agree with everything in the discussion you and David had.  This code
> is racy, but since we are scanning lockless there is no way to eliminate
> them all.  Best to just minimize the windows and document.

Agreed - and make sure that we don't have strange side. (e.g., in the 
next patch, allocate a new page, try to dissolve. Dissolving fails, what 
happens to the just-allocated page?)

--
Thanks,

David / dhildenb