On Tue, May 03, 2022 at 06:02:33PM +0200, David Hildenbrand wrote: > On 03.05.22 17:26, Minchan Kim wrote: > > 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? > > My point is that you might decide to migrate a page because you stumble > over MIGRATE_ISOLATE, although there is no need to reject long-term > pinning and to trigger page migration. > > Assume a system without ZONE_MOVABLE and without MIGRATE_CMA. Assume > someone reserves gigantic pages (alloc_contig_range()) and you have > concurrent long-term pinning on a page that is no MIGRATE_ISOLATE. > > GUP would see MIGRATE_ISOLATE and would reject pinning. The page has to > be migrated, which can fail if the page is temporarily unmovable. A dump question since I'm not familiar with hugetlb. Is above reasonable scenario? The gigantic page is about to be created using alloc_contig_range so they has MIGRATE_ISOLATE as temporal state. It means no one uses the page yet so I guess the page is not mapped at userspace but other is trying to access the page using pin_user_pages? > > See my point? We will try migrating in cases where we don't have to > migrate. I think what we would want to do is always reject pinning a CMA > page, independent of the isolation status. but we don't have that > information available. > > I raised in the past that we should look into preserving the migration > type and turning MIGRATE_ISOLATE essentially into an additional flag. > > > So I guess this patch is the right thing to do for now, but I wanted to > spell out the implications.