Re: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap_sem in munmap

From: Dave Hansen
Date: Wed Aug 22 2018 - 18:05:33 EST

On 08/22/2018 02:56 PM, owner-linux-mm@xxxxxxxxx wrote:
> On 8/22/18 2:42 PM, Dave Hansen wrote:
>> On 08/22/2018 02:10 PM, Kirill A. Shutemov wrote:
>>>> For x86, mpx_notify_unmap() looks finally zap the VM_MPX vmas in
>>>> bound table
>>>> range with zap_page_range() and doesn't update vm flags, so it
>>>> sounds ok to
>>>> me since vmas have been detached, nobody can find those vmas. But,
>>>> I'm not
>>>> familiar with the details of mpx, maybe Kirill could help to confirm
>>>> this?
>>> I don't see anything obviously dependent on down_write() in
>>> mpx_notify_unmap(), but Dave should know better.
>> We need mmap_sem for write in mpx_notify_unmap().
>> Its job is to clean up bounds tables, but bounds tables are dynamically
>> allocated and destroyed by the kernel. When we destroy a table, we also
>> destroy the VMA for the bounds table *itself*, separate from the VMA
>> being unmapped.
> Does it depends on unmap_region()? Or IOW, does it has to be called
> after unmap_region()? Now the calling sequence is:
> detach vmas
> unmap_region()
> mpx_notify_unmap()
> I'm wondering if it is safe to move it up before unmap_region() like:
> detach vmas
> mpx_notify_unmap()
> unmap_region()
> With this change we also can do our optimization to do unmap_region()
> with read mmap_sem. Otherwise it does cause problem.

I think changing the ordering is fine.

The MPX bounds table unmapping is entirely driven by the VMAs being
unmapped, so the page table unmapping in unmap_region() should not
affect it.