Re: Linux 4.18-rc7

[Hugh Dickins]

On Wed, 1 Aug 2018, Kirill A. Shutemov wrote:
> 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>

Thanks.

> 
> > 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 cannot immediately think of an absolute reason why not, but I'm
not giving it much thought; and that might turn it into a stranger
beast than it already is.

> 
> 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.

I agree it's odd, but in this respect it's no odder than /dev/zero:
that has exactly the same pattern of shmem_zero_setup() for VM_SHARED,
else vma_set_anonymous(): which made me comfortable with John's patch,
restoring the way it worked before.

Admittedly, the subsequent vfs_fallocate() might generate surprises;
and the business of doing a shmem_file_setup() first, and then undoing
it with a shmem_zero_setup(), looks weird - from John's old XXX comment,
I think it was a quick hack to piece together some functionality they
needed in a hurry, which never got revisited (they wanted a name for
the area? maybe memfd would be good for that now).

But if what's in there is working now, I do not want to mess with it:
I'd be adding bugs faster than removing them.

Hugh