Re: [PATCH v10 17/25] mm: protect mm_rb tree with a rwlock

[Laurent Dufour]

Hi Punit,

Thanks for reviewing this series.

On 30/04/2018 20:47, Punit Agrawal wrote:
> Hi Laurent,
> 
> One nitpick below.
> 
> On Tue, Apr 17, 2018 at 3:33 PM, Laurent Dufour
> <ldufour@xxxxxxxxxxxxxxxxxx> wrote:
>> This change is inspired by the Peter's proposal patch [1] which was
>> protecting the VMA using SRCU. Unfortunately, SRCU is not scaling well in
>> that particular case, and it is introducing major performance degradation
>> due to excessive scheduling operations.
>>
>> To allow access to the mm_rb tree without grabbing the mmap_sem, this patch
>> is protecting it access using a rwlock.  As the mm_rb tree is a O(log n)
>> search it is safe to protect it using such a lock.  The VMA cache is not
>> protected by the new rwlock and it should not be used without holding the
>> mmap_sem.
>>
>> To allow the picked VMA structure to be used once the rwlock is released, a
>> use count is added to the VMA structure. When the VMA is allocated it is
>> set to 1.  Each time the VMA is picked with the rwlock held its use count
>> is incremented. Each time the VMA is released it is decremented. When the
>> use count hits zero, this means that the VMA is no more used and should be
>> freed.
>>
>> This patch is preparing for 2 kind of VMA access :
>>  - as usual, under the control of the mmap_sem,
>>  - without holding the mmap_sem for the speculative page fault handler.
>>
>> Access done under the control the mmap_sem doesn't require to grab the
>> rwlock to protect read access to the mm_rb tree, but access in write must
>> be done under the protection of the rwlock too. This affects inserting and
>> removing of elements in the RB tree.
>>
>> The patch is introducing 2 new functions:
>>  - vma_get() to find a VMA based on an address by holding the new rwlock.
>>  - vma_put() to release the VMA when its no more used.
>> These services are designed to be used when access are made to the RB tree
>> without holding the mmap_sem.
>>
>> When a VMA is removed from the RB tree, its vma->vm_rb field is cleared and
>> we rely on the WMB done when releasing the rwlock to serialize the write
>> with the RMB done in a later patch to check for the VMA's validity.
>>
>> When free_vma is called, the file associated with the VMA is closed
>> immediately, but the policy and the file structure remained in used until
>> the VMA's use count reach 0, which may happens later when exiting an
>> in progress speculative page fault.
>>
>> [1] https://patchwork.kernel.org/patch/5108281/
>>
>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
>> ---
>>  include/linux/mm.h       |   1 +
>>  include/linux/mm_types.h |   4 ++
>>  kernel/fork.c            |   3 ++
>>  mm/init-mm.c             |   3 ++
>>  mm/internal.h            |   6 +++
>>  mm/mmap.c                | 115 +++++++++++++++++++++++++++++++++++------------
>>  6 files changed, 104 insertions(+), 28 deletions(-)
>>
> 
> [...]
> 
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 5601f1ef8bb9..a82950960f2e 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -160,6 +160,27 @@ void unlink_file_vma(struct vm_area_struct *vma)
>>         }
>>  }
>>
>> +static void __free_vma(struct vm_area_struct *vma)
>> +{
>> +       if (vma->vm_file)
>> +               fput(vma->vm_file);
>> +       mpol_put(vma_policy(vma));
>> +       kmem_cache_free(vm_area_cachep, vma);
>> +}
>> +
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +void put_vma(struct vm_area_struct *vma)
>> +{
>> +       if (atomic_dec_and_test(&vma->vm_ref_count))
>> +               __free_vma(vma);
>> +}
>> +#else
>> +static inline void put_vma(struct vm_area_struct *vma)
>> +{
>> +       return __free_vma(vma);
> 
> Please drop the "return".

Sure !
Thanks.

> 
> Thanks,
> Punit
> 
> [...]
>