Re: Linux 4.18-rc7

[Kirill A. Shutemov]

On Wed, Aug 01, 2018 at 11:31:52AM -0700, Hugh Dickins wrote:
> On Wed, 1 Aug 2018, Linus Torvalds wrote:
> > 
> > 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 <kirill.shutemov@xxxxxxxxxxxxxxx>

> 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