Re: Linux 4.18-rc7

[John Stultz]

On Wed, Aug 1, 2018 at 2:55 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 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,

Yes, it is weird. :)

> 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).

So my XXX comment is to do with a change I made to ashmem in order for
it to go into staging, compared with the original Android
implementation. They still carry the patch to undo it here:
https://android.googlesource.com/kernel/common.git/+/ebfc8d6476a66dc91a1b30cbfc18e67d4401af09%5E%21/

But it has more to do w/ in the shared mapping case, providing a
cleaner way of setting the vma->vm_ops = &shmem_vm_ops without having
to use shmem_zero_setup(), and doesn't change the behavior in the
private mapping case.

This discussion has spurred Joel and I to chat a bit about reviving
the effort to "properly" upstream ashmem. We're still in discussion
but I'm thinking we might just try to add the pin/unpin functionality
to memfd. We'll see how the discussion evolves.

thanks
-john