Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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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.

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



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