Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping

[Yang Shi]

Yes, this is true but I guess what Yang Shi meant was that an userspace
> > access racing with munmap is not well defined. You never know whether
> > you get your data, #PTF or SEGV because it depends on timing. The user
> > visible change might be that you lose content and get zero page instead
> > if you hit the race window while we are unmapping which was not possible
> > before. But whouldn't such an access pattern be buggy anyway? You need
> > some form of external synchronization AFAICS.
> >
> > But maybe some userspace depends on "getting right data or get SEGV"
> > semantic. If we have to preserve that then we can come up with a VM_DEAD
> > flag set before we tear it down and force the SEGV on the #PF path.
> > Something similar we already do for MMF_UNSTABLE.
>
> Set VM_DEAD with read mmap_sem held? It should be ok since this is the 
> only place to set this flag for this unique special case.

BTW, it looks the vm flags have used up in 32 bit. If we really need 
VM_DEAD, it should be for both 32-bit and 64-bit.

Any suggestion?

Thanks,
Yang

> Yang