Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after vma_merge()

[Liam Howlett]

* Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [220304 13:49]:
> * Hugh Dickins <hughd@xxxxxxxxxx> [220303 23:36]:
> > v2.6.34 commit 9d8cebd4bcd7 ("mm: fix mbind vma merge problem")
> > introduced vma_merge() to mbind_range(); but unlike madvise, mlock and
> > mprotect, it put a "continue" to next vma where its precedents go to
> > update flags on current vma before advancing: that left vma with the
> > wrong setting in the infamous vma_merge() case 8.
> > 
> > v3.10 commit 1444f92c8498 ("mm: merging memory blocks resets mempolicy")
> > tried to fix that in vma_adjust(), without fully understanding the issue.
> > 
> > v3.11 commit 3964acd0dbec ("mm: mempolicy: fix mbind_range() &&
> > vma_adjust() interaction") reverted that, and went about the fix in the
> > right way, but chose to optimize out an unnecessary mpol_dup() with a
> > prior mpol_equal() test.  But on tmpfs, that also pessimized out the
> > vital call to its ->set_policy(), leaving the new mbind unenforced.
> > 

I just thought of something after my initial email

How does the ->set_policy() requirement on tmpfs play out for the
mpol_equal() check earlier in that for loop?


> > Just delete that optimization now (though it could be made conditional
> > on vma not having a set_policy).  Also remove the "next" variable:
> > it turned out to be blameless, but also pointless.
> > 
> > Fixes: 3964acd0dbec ("mm: mempolicy: fix mbind_range() && vma_adjust() interaction")
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> > 
> >  mm/mempolicy.c | 8 +-------
> >  1 file changed, 1 insertion(+), 7 deletions(-)
> > 
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -786,7 +786,6 @@ static int vma_replace_policy(struct vm_area_struct *vma,
> >  static int mbind_range(struct mm_struct *mm, unsigned long start,
> >  		       unsigned long end, struct mempolicy *new_pol)
> >  {
> > -	struct vm_area_struct *next;
> >  	struct vm_area_struct *prev;
> >  	struct vm_area_struct *vma;
> >  	int err = 0;
> > @@ -801,8 +800,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
> >  	if (start > vma->vm_start)
> >  		prev = vma;
> >  
> > -	for (; vma && vma->vm_start < end; prev = vma, vma = next) {
> > -		next = vma->vm_next;
> > +	for (; vma && vma->vm_start < end; prev = vma, vma = vma->vm_next) {
> >  		vmstart = max(start, vma->vm_start);
> >  		vmend   = min(end, vma->vm_end);
> >  
> > @@ -817,10 +815,6 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
> >  				 anon_vma_name(vma));
> >  		if (prev) {
> >  			vma = prev;
> > -			next = vma->vm_next;
> > -			if (mpol_equal(vma_policy(vma), new_pol))
> > -				continue;
> > -			/* vma_merge() joined vma && vma->next, case 8 */
> >  			goto replace;
> >  		}
> >  		if (vma->vm_start != vmstart) {
> 
> Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>