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 | Thu, 9 Jan 2020 11:54:21 +0300 |
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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 >>> >
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