Re: [PATCH v2 1/3] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

[Kirill A. Shutemov]

On Sun, Aug 24, 2014 at 02:00:11AM +0300, Kirill A. Shutemov wrote:
> On Sat, Aug 23, 2014 at 06:11:59PM -0400, Peter Feiner wrote:
> > For VMAs that don't want write notifications, PTEs created for read
> > faults have their write bit set. If the read fault happens after
> > VM_SOFTDIRTY is cleared, then the PTE's softdirty bit will remain
> > clear after subsequent writes.
> > 
> > Here's a simple code snippet to demonstrate the bug:
> > 
> >   char* m = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
> >                  MAP_ANONYMOUS | MAP_SHARED, -1, 0);
> >   system("echo 4 > /proc/$PPID/clear_refs"); /* clear VM_SOFTDIRTY */
> >   assert(*m == '\0');     /* new PTE allows write access */
> >   assert(!soft_dirty(x));
> >   *m = 'x';               /* should dirty the page */
> >   assert(soft_dirty(x));  /* fails */
> > 
> > With this patch, write notifications are enabled when VM_SOFTDIRTY is
> > cleared. Furthermore, to avoid faults, write notifications are
> > disabled when VM_SOFTDIRTY is reset.
> > 
> > Signed-off-by: Peter Feiner <pfeiner@xxxxxxxxxx>

One more case to consider: mprotect() which doesn't trigger successful
vma_merge() will not set VM_SOFTDIRTY and will not enable write-protect on
the vma.

It's probably better to take VM_SOFTDIRTY into account in
vma_wants_writenotify() and re-think logic in other corners.

-- 
 Kirill A. Shutemov
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