linux-kernel - Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after vma

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Date:   Fri, 4 Mar 2022 18:49:33 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after
 vma_merge()

* Hugh Dickins <hughd@...gle.com> [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.
> 
> 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@...gle.com>
> ---
> 
>  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@...cle.com>