Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 21, 2018 at 09:31:22AM -0700, Yang Shi wrote:
> On 3/21/18 6:08 AM, Michal Hocko wrote:
> > Yes, this definitely sucks. One way to work that around is to split the
> > unmap to two phases. One to drop all the pages. That would only need
> > mmap_sem for read and then tear down the mapping with the mmap_sem for
> > write. This wouldn't help for parallel mmap_sem writers but those really
> > need a different approach (e.g. the range locking).
> 
> page fault might sneak in to map a page which has been unmapped before?
> 
> range locking should help a lot on manipulating small sections of a large
> mapping in parallel or multiple small mappings. It may not achieve too much
> for single large mapping.

I don't think we need range locking.  What if we do munmap this way:

Take the mmap_sem for write
Find the VMA
  If the VMA is large(*)
    Mark the VMA as deleted
    Drop the mmap_sem
    zap all of the entries
    Take the mmap_sem
  Else
    zap all of the entries
Continue finding VMAs
Drop the mmap_sem

Now we need to change everywhere which looks up a VMA to see if it needs
to care the the VMA is deleted (page faults, eg will need to SIGBUS; mmap
does not care; munmap will need to wait for the existing munmap operation
to complete), but it gives us the atomicity, at least on a per-VMA basis.

We could also do:

Take the mmap_sem for write
Mark all VMAs in the range as deleted & modify any partial VMAs
Drop mmap_sem
zap pages from deleted VMAs

That would give us the same atomicity that we have today.

Deleted VMAs would need a pointer to a completion, so operations that
need to wait can queue themselves up.  I'd recommend we use the low bit
of vm_file and treat it as a pointer to a struct completion if set.