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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Jul 03, 2018 at 10:27:18AM +0200, Michal Hocko wrote:
> On Tue 03-07-18 11:12:05, Kirill A. Shutemov wrote:
> > On Mon, Jul 02, 2018 at 02:49:28PM +0200, Michal Hocko wrote:
> > > On Mon 02-07-18 15:33:50, Kirill A. Shutemov wrote:
> > > [...]
> > > > I probably miss the explanation somewhere, but what's wrong with allowing
> > > > other thread to re-populate the VMA?
> > > 
> > > We have discussed that earlier and it boils down to how is racy access
> > > to munmap supposed to behave. Right now we have either the original
> > > content or SEGV. If we allow to simply madvise_dontneed before real
> > > unmap we could get a new page as well. There might be (quite broken I
> > > would say) user space code that would simply corrupt data silently that
> > > way.
> > 
> > Okay, so we add a lot of complexity to accommodate broken userspace that
> > may or may not exist. Is it right? :)
> 
> I would really love to do the most simple and obious thing
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 336bee8c4e25..86ffb179c3b5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2811,6 +2811,8 @@ EXPORT_SYMBOL(vm_munmap);
>  SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len)
>  {
>  	profile_munmap(addr);
> +	if (len > LARGE_NUMBER)
> +		do_madvise(addr, len, MADV_DONTNEED);
>  	return vm_munmap(addr, len);
>  }
>  
> but the argument that current semantic of good data or SEGV on
> racing threads is no longer preserved sounds valid to me. Remember
> optimizations shouldn't eat your data. How do we ensure that we won't
> corrupt data silently?

+linux-api

Frankly, I don't see change in semantics here.

Code that has race between munmap() and page fault would get intermittent
SIGSEGV before and after the approach with simple MADV_DONTNEED.

To be safe, I wouldn't go with the optimization if the process has custom
SIGSEGV handler.

> Besides that if this was so simple then we do not even need any kernel
> code. You could do that from glibc resp. any munmap wrapper. So maybe
> the proper answer is, if you do care then just help the system and
> DONTNEED your data before you munmap as an optimization for large
> mappings.

Kernel latency problems have to be handled by kernel.

-- 
 Kirill A. Shutemov