Messages in this thread | | | Subject | Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork | From | Konstantin Khlebnikov <> | Date | Fri, 10 Jan 2020 11:11:23 +0300 |
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On 10/01/2020 08.34, Wei Yang wrote: > On Fri, Jan 10, 2020 at 11:23:59AM +0800, Li Xinhai wrote: >> On 2020-01-10 at 10:30 Wei Yang wrote: >>> On Thu, Jan 09, 2020 at 11:54:21AM +0300, Konstantin Khlebnikov wrote: >>>> >>>> >>>> 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 >>>> >>> >>> Yes, your code works for DST3..DSTn. They will pick up AV1 since >>> (DST2->AV->parent == SRC3->AV). >>> >>> My question is why DST1 and DST2 has different AV? The purpose of my patch >>> tries to make child has the same topology and parent. So the ideal look of >>> child is: >>> >>> DST1 - AV1 >>> DST2 - AV1 >>> DST2 - AV1 >>> DST3 - AV1 >>> DST4 - AV1 >>> >>> Would you mind putting more words on DST1 and DST2? I didn't fully understand >>> the logic here. >>> >>> Thanks >>> >> >> I think that the first version is doing the work as you expected, but been >> revised in second version, to limits the number of users of reused old >> anon(which is picked in anon_vma_clone() and keep the tree structure. >> > > Any reason to reduce the reuse? Maybe I lost some point.
I've illustrated how two heuristics (reusing-old and sharing-prev) _could_ work together. But they both are optional.
At cloning first vma SRC1 -> DST1 there is no prev to share anon vma, thus works common code which _could_ reuse old vma because it have to.
If there is no old anon-vma which have to be reused then DST1 will allocate new anon-vma (AV1) and it will be used by DST2 and so on like on your picture.
> >>> -- >>> Wei Yang >>> Help you, Help me >
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