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mm/rmap.c: don't reuse anon_vma if we just want a copy 

Before commit 7a3ef208e6 ("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: 7a3ef208e6 ("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>
This commit is contained in:
Wei Yang 2019-11-30 17:50:56 -08:00 committed by Linus Torvalds
parent aba6dfb75f
commit 47b390d23b
1 changed files with 15 additions and 9 deletions
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24
mm/rmap.c
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@ -251,13 +251,19 @@ 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)
{
@ -287,8 +293,8 @@ 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)