linux-kernel - [PATCH] mm/mempolicy: do not duplicate policy if it is not applicable for set_mempolicy_home

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Date:   Thu, 15 Dec 2022 15:49:00 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Feng Tang <feng.tang@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Huang Ying <ying.huang@...el.com>, linux-api@...r.kernel.org,
        stable@...r.kernel.org
Subject: [PATCH] mm/mempolicy: do not duplicate policy if it is not
 applicable for set_mempolicy_home_node

On Thu 15-12-22 09:33:54, Mathieu Desnoyers wrote:
> On 2022-12-15 02:51, Michal Hocko wrote:
[...]
> > Btw. looking at the code again it seems rather pointless to duplicate
> > the policy just to throw it away anyway. A slightly bigger diff but this
> > looks more reasonable to me. What do you think? I can also send it as a
> > clean up on top of your fix.
> 
> I think it would be best if this comes as a cleanup on top of my fix. The
> diff is larger than the minimal change needed to fix the leak in stable
> branches.
> 
> Your approach looks fine, except for the vma_policy(vma) -> old change
> already spotted by Aneesh.

This shouldn't have any real effect on the functionality. Anyway, here
is a follow up cleanup:
--- 
>From f3fdb6f65fa3977aab13378b8e299b168719577c Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Thu, 15 Dec 2022 15:41:27 +0100
Subject: [PATCH] mm/mempolicy: do not duplicate policy if it is not applicable
 for set_mempolicy_home_node

set_mempolicy_home_node tries to duplicate a memory policy before
checking it whether it is applicable for the operation. There is
no real reason for doing that and it might actually be a pointless
memory allocation and deallocation exercise for MPOL_INTERLEAVE.

Not a big problem but we can do better. Simply check the policy before
acting on it.

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/mempolicy.c | 28 ++++++++++++----------------
 1 file changed, 12 insertions(+), 16 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 02c8a712282f..becf41e10076 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1489,7 +1489,7 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
-	struct mempolicy *new;
+	struct mempolicy *new, *old;
 	unsigned long vmstart;
 	unsigned long vmend;
 	unsigned long end;
@@ -1521,31 +1521,27 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
 		return 0;
 	mmap_write_lock(mm);
 	for_each_vma_range(vmi, vma, end) {
-		vmstart = max(start, vma->vm_start);
-		vmend   = min(end, vma->vm_end);
-		new = mpol_dup(vma_policy(vma));
-		if (IS_ERR(new)) {
-			err = PTR_ERR(new);
-			break;
-		}
-		/*
-		 * Only update home node if there is an existing vma policy
-		 */
-		if (!new)
-			continue;
-
 		/*
 		 * If any vma in the range got policy other than MPOL_BIND
 		 * or MPOL_PREFERRED_MANY we return error. We don't reset
 		 * the home node for vmas we already updated before.
 		 */
-		if (new->mode != MPOL_BIND && new->mode != MPOL_PREFERRED_MANY) {
-			mpol_put(new);
+		old = vma_policy(vma);
+		if (!old)
+			continue;
+		if (old->mode != MPOL_BIND && old->mode != MPOL_PREFERRED_MANY) {
 			err = -EOPNOTSUPP;
 			break;
 		}
+		new = mpol_dup(old);
+		if (IS_ERR(new)) {
+			err = PTR_ERR(new);
+			break;
+		}
 
 		new->home_node = home_node;
+		vmstart = max(start, vma->vm_start);
+		vmend   = min(end, vma->vm_end);
 		err = mbind_range(mm, vmstart, vmend, new);
 		mpol_put(new);
 		if (err)
-- 
2.30.2

-- 
Michal Hocko
SUSE Labs