On Mon, May 02, 2022 at 08:02:31PM +0200, David Hildenbrand wrote:
> On 02.05.22 19:35, Minchan Kim wrote:
> > Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA
> > so current is_pinnable_page could miss CMA pages which has MIGRATE_
> > ISOLATE. It ends up putting CMA pages longterm pinning possible on
> > pin_user_pages APIs so CMA allocation fails.
> >
> > The CMA allocation path protects the migration type change race
> > using zone->lock but what GUP path need to know is just whether the
> > page is on CMA area or not rather than exact type. Thus, we don't
> > need zone->lock but just checks the migratype in either of
> > (MIGRATE_ISOLATE and MIGRATE_CMA).
> >
> > Signed-off-by: Minchan Kim <[email protected]>
> > ---
> > include/linux/mm.h | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 6acca5cecbc5..f59bbe3296e3 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1625,8 +1625,10 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
> > #ifdef CONFIG_MIGRATION
> > static inline bool is_pinnable_page(struct page *page)
> > {
> > - return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
> > - is_zero_pfn(page_to_pfn(page));
> > + int mt = get_pageblock_migratetype(page);
> > +
> > + return !(is_zone_movable_page(page) || mt == MIGRATE_CMA ||
> > + mt == MIGRATE_ISOLATE || is_zero_pfn(page_to_pfn(page)));
> > }
> > #else
> > static inline bool is_pinnable_page(struct page *page)
>
> That implies that other memory ranges that are currently isolated
> (memory offlining, alloc_contig_range()) cannot be pinned. That might
> not be a bad thing, however, I think we could end up failing to pin
> something that's temporarily unmovable (due to temporary references).
Sure.
>
> However, I assume we have the same issue right now already with
> ZONE_MOVABLE and MIGRATE_CMA when trying to pin a page residing on these
ZONE_MOVALBE is also changed dynamically?
> there are temporarily unmovable and we fail to migrate. But it would now
> apply even without ZONE_MOVABLE or MIGRATE_CMA. Hm...
Didn't parse your last mention.
>>>> However, I assume we have the same issue right now already with
>> ZONE_MOVABLE and MIGRATE_CMA when trying to pin a page residing on these
>
> ZONE_MOVALBE is also changed dynamically?
>
Sorry, with "same issue" I meant failing to pin if having to migrate and
the page is temporarily unmovable.
>> there are temporarily unmovable and we fail to migrate. But it would now
>> apply even without ZONE_MOVABLE or MIGRATE_CMA. Hm...
>
> Didn't parse your last mention.
On a system that neither uses ZONE_MOVABLE nor MIGRATE_CMA we might have
to migrate now when pinning.
--
Thanks,
David / dhildenb
On Tue, May 03, 2022 at 03:15:24AM +0200, David Hildenbrand wrote:
>
> >>>> However, I assume we have the same issue right now already with
> >> ZONE_MOVABLE and MIGRATE_CMA when trying to pin a page residing on these
> >
> > ZONE_MOVALBE is also changed dynamically?
> >
>
> Sorry, with "same issue" I meant failing to pin if having to migrate and
> the page is temporarily unmovable.
>
> >> there are temporarily unmovable and we fail to migrate. But it would now
> >> apply even without ZONE_MOVABLE or MIGRATE_CMA. Hm...
> >
> > Didn't parse your last mention.
>
> On a system that neither uses ZONE_MOVABLE nor MIGRATE_CMA we might have
> to migrate now when pinning.
I don't understand your point. My problem is pin_user_pages with
FOLL_LONGTERM. It shouldn't pin a page from ZONE_MOVABLE and cma area
without migrating page out of movable zone or CMA area.
That's why try_grab_folio checks whether target page stays in those
movable areas. However, to check CMA area, is_migrate_cma_page is
racy so the FOLL_LONGTERM flag semantic is broken right now.
Do you see any problem of the fix?
A thing to get some attention is whether we need READ_ONCE or not
for the local variable mt.