On Mon, Dec 19, 2022 at 08:48:10AM +0000, Huang, Kai wrote: > On Mon, 2022-12-19 at 15:53 +0800, Chao Peng wrote: > > > > > > [...] > > > > > > > + > > > > + /* > > > > + * These pages are currently unmovable so don't place them into > > > > movable > > > > + * pageblocks (e.g. CMA and ZONE_MOVABLE). > > > > + */ > > > > + mapping = memfd->f_mapping; > > > > + mapping_set_unevictable(mapping); > > > > + mapping_set_gfp_mask(mapping, > > > > + mapping_gfp_mask(mapping) & ~__GFP_MOVABLE); > > > > > > But, IIUC removing __GFP_MOVABLE flag here only makes page allocation from > > > non- > > > movable zones, but doesn't necessarily prevent page from being migrated. My > > > first glance is you need to implement either a_ops->migrate_folio() or just > > > get_page() after faulting in the page to prevent. > > > > The current api restrictedmem_get_page() already does this, after the > > caller calling it, it holds a reference to the page. The caller then > > decides when to call put_page() appropriately. > > I tried to dig some history. Perhaps I am missing something, but it seems Kirill > said in v9 that this code doesn't prevent page migration, and we need to > increase page refcount in restrictedmem_get_page(): > > https://lore.kernel.org/linux-mm/20221129112139.usp6dqhbih47qpjl@xxxxxxxxxxxxxxxxx/ > > But looking at this series it seems restrictedmem_get_page() in this v10 is > identical to the one in v9 (except v10 uses 'folio' instead of 'page')? restrictedmem_get_page() increases page refcount several versions ago so no change in v10 is needed. You probably missed my reply: https://lore.kernel.org/linux-mm/20221129135844.GA902164@xxxxxxxxxxxxxxxxxx/ The current solution is clear: unless we have better approach, we will let restrictedmem user (KVM in this case) to hold the refcount to prevent page migration. Thanks, Chao > > Anyway if this is not fixed, then it should be fixed. Otherwise, a comment at > the place where page refcount is increased will be helpful to help people > understand page migration is actually prevented. >
