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

Wei Yang
2020-01-10T05:34:50Z

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.

>>--
>>Wei Yang
>>Help you, Help me

--
Wei Yang
Help you, Help me