Re: [RFC PATCH RESEND 10/28] mm/mmap: mark VMAs as locked in vma_adjust

[Laurent Dufour]

Le 09/09/2022 à 02:51, Suren Baghdasaryan a écrit :
> On Tue, Sep 6, 2022 at 8:35 AM Laurent Dufour <ldufour@xxxxxxxxxxxxx> wrote:
>>
>> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
>>> vma_adjust modifies a VMA and possibly its neighbors. Mark them as locked
>>> before making the modifications.
>>>
>>> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>>> ---
>>>  mm/mmap.c | 11 ++++++++++-
>>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>> index f89c9b058105..ed58cf0689b2 100644
>>> --- a/mm/mmap.c
>>> +++ b/mm/mmap.c
>>> @@ -710,6 +710,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>>       long adjust_next = 0;
>>>       int remove_next = 0;
>>>
>>> +     vma_mark_locked(vma);
>>> +     if (next)
>>> +             vma_mark_locked(next);
>>> +
>>
>> I was wondering if the VMAs insert and expand should be locked too.
>>
>> For expand, I can't see any valid reason, but for insert, I'm puzzled.
>> I would think that it is better to lock the VMA to be inserted but I can't
>> really justify that.
>>
>> It may be nice to detail why this is not need to lock insert and expand here.
> 
> 'expand' is always locked before it's passed to __vma_adjust() by
> vma_merge(). It has to be locked before we decide "Can it merge with
> the predecessor?" here
> https://elixir.bootlin.com/linux/latest/source/mm/mmap.c#L1201 because
> a change in VMA can affect that decision. I spent many hours tracking
> the issue caused by not locking the VMA before making this decision.
> It might be good to add a comment about this...
> 
> AFAIKT 'insert' is only used by __split_vma() and it's always a brand
> new VMA which is not yet linked into mm->mmap. Any reason
> __vma_adjust() should lock it?

No, I think that's good this way.

> 
>>
>>>       if (next && !insert) {
>>>               struct vm_area_struct *exporter = NULL, *importer = NULL;
>>>
>>> @@ -754,8 +758,11 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>>                        * If next doesn't have anon_vma, import from vma after
>>>                        * next, if the vma overlaps with it.
>>>                        */
>>> -                     if (remove_next == 2 && !next->anon_vma)
>>> +                     if (remove_next == 2 && !next->anon_vma) {
>>>                               exporter = next->vm_next;
>>> +                             if (exporter)
>>> +                                     vma_mark_locked(exporter);
>>> +                     }
>>>
>>>               } else if (end > next->vm_start) {
>>>                       /*
>>> @@ -931,6 +938,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>>                        * "vma->vm_next" gap must be updated.
>>>                        */
>>>                       next = vma->vm_next;
>>> +                     if (next)
>>> +                             vma_mark_locked(next);
>>>               } else {
>>>                       /*
>>>                        * For the scope of the comment "next" and
>>
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