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
