Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork

lixinhai.lxh
2020-01-08T06:42:32Z

On 2020-01-08 at 10:32 Wei Yang wrote:
>On Tue, Jan 07, 2020 at 01:19:56PM +0300, Konstantin Khlebnikov wrote:
>>This fixes some misconceptions in commit 4e4a9eb92133 ("mm/rmap.c: reuse
>>mergeable anon_vma as parent when fork"). It merges anon-vma in unexpected
>>way but fortunately still produces valid anon-vma tree, so nothing crashes.
>>
>>If in parent VMAs: SRC1 SRC2 .. SRCn share anon-vma ANON0, then after fork
>>before all patches in child process related VMAs: DST1 DST2 .. DSTn will
>>fork indepndent anon-vmas: ANON1 ANON2 .. ANONn (each is child of ANON0).
>>Before this patch only DST1 will fork new ANON1 and following DST2 .. DSTn
>>will share parent's ANON0 (i.e. anon-vma tree is valid but isn't optimal).
>>With this patch DST1 will create new ANON1 and DST2 .. DSTn will share it.
>>
>>Root problem caused by initialization order in dup_mmap(): vma->vm_prev
>>is set after calling anon_vma_fork(). Thus in anon_vma_fork() it points to
>>previous VMA in parent mm.
>>
>>Second problem is hidden behind first one: assumption "Parent has vm_prev,
>>which implies we have vm_prev" is wrong if first VMA in parent mm has set
>>flag VM_DONTCOPY. Luckily prev->anon_vma doesn't dereference NULL pointer
>>because in current code 'prev' actually is same as 'pprev'.
>>
>>Third hidden problem is linking between VMA and anon-vmas whose pages it
>>could contain. Loop in anon_vma_clone() attaches only parent's anon-vmas,
>>shared anon-vma isn't attached. But every mapped page stays reachable in
>>rmap because we erroneously share anon-vma from parent's previous VMA.
>>
>>This patch moves sharing logic out of anon_vma_clone() into more specific
>>anon_vma_fork() because this supposed to work only at fork() and simply
>>reuses anon_vma from previous VMA if it is forked from the same anon-vma.
>>
>>Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>>Reported-by: Li Xinhai <lixinhai.lxh@gmail.com>
>>Fixes: 4e4a9eb92133 ("mm/rmap.c: reuse mergeable anon_vma as parent when fork")
>>Link: https://lore.kernel.org/linux-mm/CALYGNiNzz+dxHX0g5-gNypUQc3B=8_Scp53-NTOh=zWsdUuHAw@mail.gmail.com/T/#t
>>---
>> include/linux/rmap.h |    3 ++-
>> kernel/fork.c        |    2 +-
>> mm/rmap.c            |   23 +++++++++--------------
>> 3 files changed, 12 insertions(+), 16 deletions(-)
>>
>>diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>index 988d176472df..560e4480dcd0 100644
>>--- a/include/linux/rmap.h
>>+++ b/include/linux/rmap.h
>>@@ -143,7 +143,8 @@ void anon_vma_init(void); /* create anon_vma_cachep */
>> int  __anon_vma_prepare(struct vm_area_struct *);
>> void unlink_anon_vmas(struct vm_area_struct *);
>> int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *);
>>-int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
>>+int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma,
>>+   struct vm_area_struct *prev);
>>
>> static inline int anon_vma_prepare(struct vm_area_struct *vma)
>> {
>>diff --git a/kernel/fork.c b/kernel/fork.c
>>index 2508a4f238a3..c33626993831 100644
>>--- a/kernel/fork.c
>>+++ b/kernel/fork.c
>>@@ -556,7 +556,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>> tmp->anon_vma = NULL;
>> if (anon_vma_prepare(tmp))
>> goto fail_nomem_anon_vma_fork;
>>- } else if (anon_vma_fork(tmp, mpnt))
>>+ } else if (anon_vma_fork(tmp, mpnt, prev))
>> goto fail_nomem_anon_vma_fork;
>> tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
>> tmp->vm_next = tmp->vm_prev = NULL;
>>diff --git a/mm/rmap.c b/mm/rmap.c
>>index b3e381919835..3c1e04389291 100644
>>--- a/mm/rmap.c
>>+++ b/mm/rmap.c
>>@@ -269,19 +269,6 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>> {
>> struct anon_vma_chain *avc, *pavc;
>> struct anon_vma *root = NULL;
>>- struct vm_area_struct *prev = dst->vm_prev, *pprev = src->vm_prev;
>>-
>>- /*
>>- * If parent share anon_vma with its vm_prev, keep this sharing in in
>>- * child.
>>- *
>>- * 1. Parent has vm_prev, which implies we have vm_prev.
>>- * 2. Parent and its vm_prev have the same anon_vma.
>>- */
>>- if (!dst->anon_vma && src->anon_vma &&
>>-     pprev && pprev->anon_vma == src->anon_vma)
>>- dst->anon_vma = prev->anon_vma;
>>-
>>
>> list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
>> struct anon_vma *anon_vma;
>>@@ -332,7 +319,8 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>>  * the corresponding VMA in the parent process is attached to.
>>  * Returns 0 on success, non-zero on failure.
>>  */
>>-int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>+int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma,
>>+   struct vm_area_struct *prev)
>> {
>> struct anon_vma_chain *avc;
>> struct anon_vma *anon_vma;
>>@@ -342,6 +330,13 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>> if (!pvma->anon_vma)
>> return 0;
>>
>>+ /* Share anon_vma with previous VMA if it has the same parent. */
>>+ if (prev && prev->anon_vma &&
>>+     prev->anon_vma->parent == pvma->anon_vma) {
>>+ vma->anon_vma = prev->anon_vma;
>>+ return anon_vma_clone(vma, prev);
>>+ }
>>+
>

the checking in first version has done right thing, or anything missed here?
+ if (pvma->anon_vma && pprev && prev &&
+    pprev->anon_vma == pvma->anon_vma &&
+    pprev->vm_start == prev->vm_start)
except that 'pvma->anon_vma' has been checked at the begining of this function,
it can be removed from this block.

>I am afraid this one change the intended behavior. Let's put a chart to
>describe.
>
>Commit 4e4a9eb92133 ("mm/rmap.c: reusemergeable anon_vma as parent when
>fork") tries to improve the following situation.
>
>Before the commit, the behavior is like this:
>
>Parent process:
>
>      +-----+
>      | pav |<-----------------+----------------------+
>      +-----+                  |                      |
>                               |                      |
>                   +-----------+          +-----------+
>                   |pprev      |          |pvma       |
>                   +-----------+          +-----------+
>
>Child Process
>
>
>      +-----+                     +-----+
>      | av1 |<-----------------+  | av2 |<------------+
>      +-----+                  |  +-----+             |
>                               |                      |
>                   +-----------+          +-----------+
>                   |prev       |          |vma        |
>                   +-----------+          +-----------+
>
>
>Parent pprev and pvma share the same anon_vma due to
>find_mergeable_anon_vma(). While the anon_vma_clone() would pick up different
>anon_vma for child process's vma.
>
>The purpose of my commit is to give child process the following shape.
>
>      +-----+
>      | av  |<-----------------+----------------------+
>      +-----+                  |                      |
>                               |                      |
>                   +-----------+          +-----------+
>                   |prev       |          |vma        |
>                   +-----------+          +-----------+
>
>After this, we reduce the extra "av2" for child process. But yes, because of
>the two reasons you found, it didn't do the exact thing.
>
>While if my understanding is correct, the anon_vma_clone() would pick up any
>anon_vma in its process tree, except parent's. If this fails to get a reusable
>one, anon_vma_fork() would allocate one, whose parent is pvma->anon_vma.
>
>Let me summarise original behavior:
>
>  * if anon_vma_clone succeed, it find one anon_vma in the process tree, but
>    it could not be pvma->anon_vma
>  * if anon_vma_clone fail, it will allocate a new anon_vma and its parent is
>    pvma->anon_vam
>
>Then take a look into your code here.
>
>"prev->anon_vma->parent == pvma->anon_vma" means prev->anon_vma parent is
>pvma's anon_vma. If my understanding is correct, this just match the second
>case. For "prev", we didn't find a reusable anon_vma and allocate a new one.
>
>But how about the first case? prev reuse an anon_vma in the process tree which
>is not parent's?
>
>> /* Drop inherited anon_vma, we'll reuse existing or allocate new. */
>> vma->anon_vma = NULL;
>>
>
>--
>Wei Yang
>Help you, Help me