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

[Kirill A. Shutemov]

On Wed, Aug 22, 2018 at 01:45:44PM -0700, Yang Shi wrote:
> 
> 
> On 8/22/18 4:19 AM, Vlastimil Babka wrote:
> > On 08/15/2018 08:49 PM, Yang Shi wrote:
> > > +	downgrade_write(&mm->mmap_sem);
> > > +
> > > +	/* Zap mappings with read mmap_sem */
> > > +	unmap_region(mm, start_vma, prev, start, end);
> > > +
> > > +	arch_unmap(mm, start_vma, start, end);
> > Hmm, did you check that all architectures' arch_unmap() is safe with
> > read mmap_sem instead of write mmap_sem? E.g. x86 does
> > mpx_notify_unmap() there where I would be far from sure at first glance...
> 
> Yes, I'm also not quite sure if it is 100% safe or not. I was trying to move
> this before downgrade_write, however, I'm not sure if it is ok or not too,
> so I keep the calling sequence.
> 
> For architectures, just x86 and ppc really do something. PPC just uses it
> for vdso unmap which should just happen during process exit, so it sounds
> safe.
> 
> 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.

-- 
 Kirill A. Shutemov