Re: [RFC][PATCH 3/6] mm: VMA sequence count

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

On Thu, Oct 23, 2014 at 04:22:24PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 23, 2014 at 03:36:16PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Oct 22, 2014 at 03:44:16PM +0200, Peter Zijlstra wrote:
> > > On Wed, Oct 22, 2014 at 02:15:54PM +0200, Peter Zijlstra wrote:
> > > > On Wed, Oct 22, 2014 at 02:53:04PM +0300, Kirill A. Shutemov wrote:
> > > > > Em, no. In this case change_protection() will not touch the pte, since
> > > > > it's pte_none() and the pte_same() check will pass just fine.
> > > > 
> > > > Oh, that's what you meant. Yes that's a problem, yes vm_page_prot
> > > > needs wrapping too.
> > > 
> > > Maybe also vm_policy, is there anything else that can change while a vma
> > > lives?
> > 
> >  - vm_flags, obviously;
> 
> Do those ever change?

The flags which can change (probably incomplete):

 - prot-related: VM_READ, VM_WRITE, VM_EXEC -- mprotect();
 - VM_LOCKED - mlock();
 - VM_SEQ_READ, VM_RAND_READ, VM_DONTCOPY, VM_DONTDUMP, VM_HUGEPAGE,
   VM_NOHUGEPAGE, VM_MERGEABLE -- madvise();
 - VM_SOFTDIRTY -- through procfs;
 
> The only thing that jumps out is the VM_LOCKED thing and that should not
> really matter one way or the other, but sure can do.

I would not be that sure about VM_LOCKED. Consider munlock() vs. write
fault race.

static int do_wp_page(struct fault_env *fe)
        __releases(ptl)
{
...
err:
	if (old_page) {
		/*
		 * Don't let another task, with possibly unlocked vma,
		 * keep the mlocked page.
		 */
		if ((ret & VM_FAULT_WRITE) && (fe->vma->vm_flags & VM_LOCKED)) {
			lock_page(old_page);	/* LRU manipulation */
			munlock_vma_page(old_page);
			unlock_page(old_page);
		}
		page_cache_release(old_page);
	}
	return ret;
...
}

The page can leak out mlocked, iiuc.

Some other flags can be problematic too.

> In any case, yes I'll go include them.

I hope it will not hurt single-threaded workloads even more. :-/

-- 
 Kirill A. Shutemov

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