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

Konstantin Khlebnikov
2020-01-09T08:54:29Z

On 09/01/2020 05.52, Wei Yang wrote:
> On Wed, Jan 08, 2020 at 01:40:44PM +0300, Konstantin Khlebnikov wrote:
>> On 08/01/2020 05.32, 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);
>>>> + }
>>>> +
>>>
>>> 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?
>>
>> If anon_vma_clone() pick old anon-vma for first vma in sharing chain (prev)
>> then second vma (vma) will fork new anon-vma (unless pick another old anon-vma),
>> then third vma will share it. And so on.
>
> No, I am afraid you are not correct here. Or I don't understand your sentence.
>
> This is my understanding about the behavior before my commit. Suppose av1 and
> av2 are both reused from old anon_vma. And if my understanding is correct,
> they are different from pvma->anon_vma. Then how your code match this
> situatioin?
>
> +-----+ +-----+
> | av1 |<-----------------+ | av2 |<------------+
> +-----+ | +-----+ |
> | |
> +-----------+ +-----------+
> |prev | |vma |
> +-----------+ +-----------+
>
> Would you explain your understanding the second and third vma in your
> sentence? Which case you are trying to illustrate?

series of vma in parent with shared AV:

SRC1 - AV0
SRC2 - AV0
SRC3 - AV0
...
SRCn - AV0

in child after fork

DST1 - AV_OLD_1 (some old vma, picked by anon_vma_clone) plus DST1 is attached to same AVs as SRC1
DST2 - AV_OLD_2 (other old vma) plus DST1 is attached to same AVs as SRC2
DST2 - AV1 prev AV parent does not match AV0, no old vma found for reusing -> allocate new one (child of AV0)
DST3 - AV1 - DST2->AV->parent == SRC3->AV (AV0) -> share AV with prev
DST4 - AV1 - same thing
...
DSTn - AV1

>
>> Fork works left to right - we don't known about next vma to predict sharing and
>> choose better options.
>>
>> But reusing old vma doesn't allocates new one. It's better to not reuse them
>
> You mean reuse old anon_vma here?

Picking ancestor AV in anon_vma_clone instead of allocating new one.

>
>> second time because this makes tree less optimal (and actually not a tree anymore).
>> This is just a trick to prevent unlimited growth anon-vma chains in background:
>> while each anon-vma has at least one vma or two childs then their count is
>> limited with count of vmas which are visible and limited.
>>
>>>
>>>> /* Drop inherited anon_vma, we'll reuse existing or allocate new. */
>>>> vma->anon_vma = NULL;
>>>>
>>>
>>> --
>>> Wei Yang
>>> Help you, Help me
>>>
>