Re: [PATCH v5 2/7] mm/mremap: Allow moves within the same VMA

[Joel Fernandes <joel@joelfernandes.org>]

On Sun, Aug 27, 2023 at 10:21:14AM +0100, Lorenzo Stoakes wrote:
[..] 
> >
> >  /*
> >   * Flags used by change_protection().  For now we make it a bitmap so
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 035fbf542a8f..06baa13bd2c8 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -490,12 +490,13 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
> >  }
> >
> >  /*
> > - * A helper to check if a previous mapping exists. Required for
> > - * move_page_tables() and realign_addr() to determine if a previous mapping
> > - * exists before we can do realignment optimizations.
> > + * A helper to check if aligning down is OK. The aligned address should fall
> > + * on *no mapping*. For the stack moving down, that's a special move within
> > + * the VMA that is created to span the source and destination of the move,
> > + * so we make an exception for it.
> >   */
> >  static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_align,
> > -			       unsigned long mask)
> > +			    unsigned long mask, bool for_stack)
> >  {
> >  	unsigned long addr_masked = addr_to_align & mask;
> >
> > @@ -504,7 +505,7 @@ static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_ali
> >  	 * of the corresponding VMA, we can't align down or we will destroy part
> >  	 * of the current mapping.
> >  	 */
> > -	if (vma->vm_start != addr_to_align)
> > +	if (!for_stack && vma->vm_start != addr_to_align)
> >  		return false;
> 
> I'm a little confused by this exception, is it very specifically for the
> shift_arg_pages() case where can assume we are safe to just discard the
> lower portion of the stack?
> 
> Wouldn't the find_vma_intersection() line below fail in this case? I may be
> missing something here :)

I think you are right. In v4, this was not an issue as we did this:


+	if (!for_stack && vma->vm_start != addr_to_align)
+		return false;
+
+	cur = find_vma_prev(vma->vm_mm, vma->vm_start, &prev);
+	if (WARN_ON_ONCE(cur != vma))
+		return false;

Which essentially means this patch is a NOOP in v5 for the stack case.

So what we really want is the VMA previous to @vma and whether than subsumes
the masked address.

Should I just change it back to the v4 version then as above for both patch 1
and 2 and carry your review tags?

This is also hard to test as it requires triggering the execve stack move
case. Though it is not a bug (as it is essentially a NOOP), it still would be
nice to test it. This is complicated by also the fact that mremap(2) itself
does not allow overlapping moves. I could try to hardcode the unfavorable
situation as I have done in the past to force that mremap warning.

thanks,

 - Joel