From 58d689205be3679747fc298e5bc07711d6e48389 Mon Sep 17 00:00:00 2001
From: Wei Yang <richardw.yang@linux.intel.com>
Date: Sat, 30 Nov 2019 17:50:56 -0800
Subject: [PATCH] mm/rmap.c: don't reuse anon_vma if we just want a copy
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References: git-fixes, bsc#1203098
Patch-mainline: v5.5
Git-commit: 47b390d23bf81894395c8773acf6f73c66465dc4
Before commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma
hierarchy"), anon_vma_clone() doesn't change dst->anon_vma. While after
this commit, anon_vma_clone() will try to reuse an exist one on forking.
But this commit go a little bit further for the case not forking.
anon_vma_clone() is called from __vma_split(), __split_vma(), copy_vma()
and anon_vma_fork(). For the first three places, the purpose here is
get a copy of src and we don't expect to touch dst->anon_vma even it is
NULL.
While after that commit, it is possible to reuse an anon_vma when
dst->anon_vma is NULL. This is not we intend to have.
This patch stops reuse of anon_vma for non-fork cases.
Link: http://lkml.kernel.org/r/20191011072256.16275-1-richardw.yang@linux.intel.com
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Qian Cai <cai@lca.pw>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/rmap.c | 24 +++++++++++++++---------
1 file changed, 15 insertions(+), 9 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index e82a64a6b736..84dfe563c759 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -250,13 +250,19 @@ static inline void unlock_anon_vma_root(struct anon_vma *root)
* Attach the anon_vmas from src to dst.
* Returns 0 on success, -ENOMEM on failure.
*
- * If dst->anon_vma is NULL this function tries to find and reuse existing
- * anon_vma which has no vmas and only one child anon_vma. This prevents
- * degradation of anon_vma hierarchy to endless linear chain in case of
- * constantly forking task. On the other hand, an anon_vma with more than one
- * child isn't reused even if there was no alive vma, thus rmap walker has a
- * good chance of avoiding scanning the whole hierarchy when it searches where
- * page is mapped.
+ * anon_vma_clone() is called by __vma_split(), __split_vma(), copy_vma() and
+ * anon_vma_fork(). The first three want an exact copy of src, while the last
+ * one, anon_vma_fork(), may try to reuse an existing anon_vma to prevent
+ * endless growth of anon_vma. Since dst->anon_vma is set to NULL before call,
+ * we can identify this case by checking (!dst->anon_vma && src->anon_vma).
+ *
+ * If (!dst->anon_vma && src->anon_vma) is true, this function tries to find
+ * and reuse existing anon_vma which has no vmas and only one child anon_vma.
+ * This prevents degradation of anon_vma hierarchy to endless linear chain in
+ * case of constantly forking task. On the other hand, an anon_vma with more
+ * than one child isn't reused even if there was no alive vma, thus rmap
+ * walker has a good chance of avoiding scanning the whole hierarchy when it
+ * searches where page is mapped.
*/
int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
{
@@ -286,8 +292,8 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
* will always reuse it. Root anon_vma is never reused:
* it has self-parent reference and at least one child.
*/
- if (!dst->anon_vma && anon_vma != src->anon_vma &&
- anon_vma->degree < 2)
+ if (!dst->anon_vma && src->anon_vma &&
+ anon_vma != src->anon_vma && anon_vma->degree < 2)
dst->anon_vma = anon_vma;
}
if (dst->anon_vma)