Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping

[Yang Shi]

On 6/29/18 9:26 PM, Yang Shi wrote:
> On 6/29/18 8:15 PM, Andrew Morton wrote:
> > On Fri, 29 Jun 2018 19:28:15 -0700 Yang Shi 
> > <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
> >
> > > > we're adding a bunch of code to 32-bit kernels which will never be
> > > > executed.
> > > >
> > > > I'm thinking it would be better to be much more explicit with "#ifdef
> > > > CONFIG_64BIT" in this code, rather than relying upon the above magic.
> > > >
> > > > But I tend to think that the fact that we haven't solved anything on
> > > > locked vmas or on uprobed mappings is a shostopper for the whole
> > > > approach :(
> > > I agree it is not that perfect. But, it still could improve the most 
> > > use
> > > cases.
> > Well, those unaddressed usecases will need to be fixed at some point.
>
> Yes, definitely.
>
> > What's our plan for that?
>
> As I mentioned in the earlier email, locked and hugetlb cases might be 
> able to be solved by separating vm_flags update and actual unmap. I 
> will look into it further later.

By looking into this furtherï I think both mlocked and hugetlb vmas can 
be handled.

For mlocked vmas, it is easy since we acquires write mmap_sem before 
unmapping, so VM_LOCK flags can be cleared here then unmap, just like 
what the regular path does.

For hugetlb vmas, the VM_MAYSHARE flag is just checked by 
huge_pmd_share() in hugetlb_fault()->huge_pte_alloc(), another call site 
is dup_mm()->copy_page_range()->copy_hugetlb_page_range(), we don't care 
this call chain in this case.

So we may expand VM_DEAD to hugetlb_fault(). Michal suggested to check 
VM_DEAD in check_stable_address_space(), so it would be called in 
hugetlb_fault() too (not in current code), then the page fault handler 
would bail out before huge_pte_alloc() is called.

With this trick, we don't have to care about when the vm_flags is 
updated, we can unmap hugetlb vmas in read mmap_sem critical section, 
then update the vm_flags with write mmap_sem held or before the unmap.

Yang

> From my point of view, uprobe mapping sounds not that vital.
>
> > Would one of your earlier designs have addressed all usecases? I
> > expect the dumb unmap-a-little-bit-at-a-time approach would have?
>
> Yes. The v1 design does unmap with holding write map_sem. So, the 
> vm_flags update is not a problem.
>
> Thanks,
> Yang
>
> > > For the locked vmas and hugetlb vmas, unmapping operations need modify
> > > vm_flags. But, I'm wondering we might be able to separate unmap and
> > > vm_flags update. Because we know they will be unmapped right away, the
> > > vm_flags might be able to be updated in write mmap_sem critical section
> > > before the actual unmap is called or after it. This is just off the top
> > > of my head.
> > >
> > > For uprobed mappings, I'm not sure how vital it is to this case.
> > >
> > > Thanks,
> > > Yang