Felix Kuehling <felix.kuehling@xxxxxxx> writes: > Am 2022-02-16 um 07:26 schrieb Jason Gunthorpe: >> The other place that needs careful audit is all the callers using >> vm_normal_page() - they must all be able to accept a ZONE_DEVICE page >> if we don't set pte_devmap. > > How much code are we talking about here? A quick search finds 26 call-sites in > 12 files in current master: > > fs/proc/task_mmu.c > mm/hmm.c > mm/gup.c > mm/huge_memory.c (vm_normal_page_pmd) > mm/khugepaged.c > mm/madvise.c > mm/mempolicy.c > mm/memory.c > mm/mlock.c > mm/migrate.c > mm/mprotect.c > mm/memcontrol.c > > I'm thinking of a more theoretical approach: Instead of auditing all users, I'd > ask, what are the invariants that a vm_normal_page should have. Then check, > whether our DEVICE_COHERENT pages satisfy them. But maybe the concept of a > vm_normal_page isn't defined clearly enough for that. > > That said, I think we (Alex and myself) made an implicit assumption from the > start, that a DEVICE_COHERENT page should behave a lot like a normal page in > terms of VMA mappings, even if we didn't know what that means in detail. Yes I'm afraid I made a similar mistake when reviewing this, forgetting that DEVICE_COHERENT pages are not LRU pages and therefore need special treatment in some places. So for now I will have to withdraw my reviewed-by until this has been looked at more closely, because as you note below accidentally treating them as LRU pages leads to a bad time. > I can now at least name some differences between DEVICE_COHERENT and normal > pages: how the memory is allocated, how data is migrated into DEVICE_COHERENT > pages and that it can't be on any LRU list (because the lru list_head in struct > page is aliased by pgmap and zone_device_data). Maybe I'll find more differences > if I keep digging. > > Regards, > Felix > > >> >> Jason
