On Wed, Aug 01, 2018 at 11:31:52AM -0700, Hugh Dickins wrote:

On Wed, 1 Aug 2018, Linus Torvalds wrote:

quoted

Anyway, the upshot of all this is that I think I know what the ia64
problem was, and John sent the patch for the ashmem case, and I'm
going to hold off reverting that vma_is_anonymous() false-positives
commit after all.

I'd better send deletion of zap_pmd_range()'s VM_BUG_ON_VMA(): below
(but I've no proprietorial interest, if you prefer to do your own).

Agreed.

Acked-by: Kirill A. Shutemov <redacted>

John's patch is good, and originally I thought it was safe from that
VM_BUG_ON_VMA(), because the /dev/ashmem fd exposed to the user is
disconnected from the vm_file in the vma, and madvise(,,MADV_REMOVE)
insists on VM_SHARED. But afterwards read John's earlier mail,
drawing attention to the vfs_fallocate() in there: I may be wrong,
and I don't know if Android has THP in the config anyway, but it looks
to me like an unmap_mapping_range() from ashmem's vfs_fallocate()
could hit precisely the VM_BUG_ON_VMA(), once it's vma_is_anonymous().

(I'm not familiar with ashmem, and I certainly don't understand the
role of MAP_PRIVATE ashmem mappings - hole-punch's zap_pte_range()
should end up leaving any anon pages in place; but the presence of
the BUG is requiring us all to understand too much too quickly.)

Hugh, do you see any reason why ashmem shouldn't have vm_ops ==
shmem_vm_ops?

I don't understand ashmem, but I feel uncomfortable that we have this
sneaky way to create an anonymous VMA. It feels wrong to me.

-- 
 Kirill A. Shutemov