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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote:
> On 3/21/18 10:29 AM, Matthew Wilcox wrote:
> > 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
> 
> Marking vma as deleted sounds good. The problem for my current approach is
> the concurrent page fault may succeed if it access the not yet unmapped
> section. Marking deleted vma could tell page fault the vma is not valid
> anymore, then return SIGSEGV.
> 
> > does not care; munmap will need to wait for the existing munmap operation
> 
> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right?

Oh, I forgot about MAP_FIXED.  Yes, MAP_FIXED should wait for the munmap
to finish.  But a regular mmap can just pretend that it happened before
the munmap call and avoid the deleted VMAs.