Commit 47b390d2 authored by Wei Yang's avatar Wei Yang Committed by Linus Torvalds
Browse files

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

Before commit 7a3ef208 ("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: 7a3ef208 ("mm: prevent endless growth of anon_vma hierarchy")
Signed-off-by: default avatarWei Yang <richardw.yang@linux.intel.com>
Acked-by: default avatarKonstantin 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent aba6dfb7

mm/rmap.c

+15 −9
Original line number Diff line number Diff line
@@ -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)

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