Re: [PATCH] mm/rmap: fix reusing mergeable anon_vma as parent when fork

["lixinhai.lxh@gmail.com" <lixinhai.lxh@gmail.com>]

On 2020-01-07 at 04:35 Konstantin Khlebnikov wrote:
>On Mon, Jan 6, 2020 at 1:42 PM Konstantin Khlebnikov
><khlebnikov@yandex-team.ru> wrote:
>>
>> This fixes couple misconceptions in commit 4e4a9eb92133 ("mm/rmap.c: reuse
>> mergeable anon_vma as parent when fork").
>>
>> First 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. This is fixed by rearrangement in dup_mmap().
>>
>> 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
>> use different anon-vmas: ANON1 ANON2 .. ANONn. Before this patch only DST1
>> will fork new ANON1 and following DST2 .. DSTn will share parent's ANON0.
>> With this patch DST1 will create new ANON1 and DST2 .. DSTn will share it.
>>
>> Also this patch moves sharing logic out of anon_vma_clone() into more
>> specific anon_vma_fork() because this supposed to work only at fork().
>> Function anon_vma_clone() is more generic is also used at splitting VMAs.
>>
>> 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'. To avoid that
>> this patch just checks pointer and compares vm_start to verify relation
>> between previous VMAs in parent and child.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> Fixes: 4e4a9eb92133 ("mm/rmap.c: reuse mergeable anon_vma as parent when fork")
>
>Oops, I've forgot to mention that Li Xinhai <lixinhai.lxh@gmail.com>
>found and reported this suspicious code. Sorry.
>
>Reported-by: Li Xinhai <lixinhai.lxh@gmail.com>
>Link: https://lore.kernel.org/linux-mm/CALYGNiNzz+dxHX0g5-gNypUQc3B=8_Scp53-NTOh=zWsdUuHAw@mail.gmail.com/T/#t
>

Can we change the interface 
int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma),
to
int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma, struct vm_area_struct *pcvma),
and 'pcvma' means previous child vma.
so highlight the use of that vma, and the current code sequence for linking 'tmp' vma
in dup_mmap() is not changed(in case some code would have dependency on that
linking sequence)

Another issue is for linking the avc for the reused anon_vma. anon_vma_clone()
use the iteration
list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma),
to link avc for child vma, and it is unable to reach the resued anon_vma because
that is from the previous vma not from parent vma. So, in anon_vma_fork(), 
we need to setup the avc link for vma->anon.

>> ---
>>  kernel/fork.c |    4 ++--
>>  mm/rmap.c     |   25 ++++++++++++-------------
>>  2 files changed, 14 insertions(+), 15 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 2508a4f238a3..04ee5e243f65 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -548,6 +548,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>>                 if (retval)
>>                         goto fail_nomem_policy;
>>                 tmp->vm_mm = mm;
>> +               tmp->vm_prev = prev;    /* anon_vma_fork use this */
>> +               tmp->vm_next = NULL;
>>                 retval = dup_userfaultfd(tmp, &uf);
>>                 if (retval)
>>                         goto fail_nomem_anon_vma_fork;
>> @@ -559,7 +561,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>>                 } else if (anon_vma_fork(tmp, mpnt))
>>                         goto fail_nomem_anon_vma_fork;
>>                 tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
>> -               tmp->vm_next = tmp->vm_prev = NULL;
>>                 file = tmp->vm_file;
>>                 if (file) {
>>                         struct inode *inode = file_inode(file);
>> @@ -592,7 +593,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>>                  */
>>                 *pprev = tmp;
>>                 pprev = &tmp->vm_next;
>> -               tmp->vm_prev = prev;
>>                 prev = tmp;
>>
>>                 __vma_link_rb(mm, tmp, rb_link, rb_parent);
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index b3e381919835..77b3aa38d5c2 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;
>> @@ -334,6 +321,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>>   */
>>  int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>  {
>> +       struct vm_area_struct *prev = vma->vm_prev, *pprev = pvma->vm_prev;
>>         struct anon_vma_chain *avc;
>>         struct anon_vma *anon_vma;
>>         int error;
>> @@ -345,6 +333,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>>         /* Drop inherited anon_vma, we'll reuse existing or allocate new. */
>>         vma->anon_vma = NULL;
>>
>> +       /*
>> +        * If parent shares anon_vma with its vm_prev, keep this sharing.
>> +        *
>> +        * Previous VMA could be missing or not match previuos in parent
>> +        * if VM_DONTCOPY is set: compare vm_start to avoid this case.
>> +        */
>> +       if (pvma->anon_vma && pprev && prev &&
>> +           pprev->anon_vma == pvma->anon_vma &&
>> +           pprev->vm_start == prev->vm_start)
>> +               vma->anon_vma = prev->anon_vma;
>> +
>>         /*
>>          * First, attach the new VMA to the parent VMA's anon_vmas,
>>          * so rmap can find non-COWed pages in child processes.
>>
>>