Re: [PATCH v9 17/24] mm: Protect mm_rb tree with a rwlock

[Laurent Dufour]

On 03/04/2018 02:11, David Rientjes wrote:
> On Tue, 13 Mar 2018, Laurent Dufour 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>
> 
> Can __free_vma() be generalized for mm/nommu.c's delete_vma() and 
> do_mmap()?

Good question !
I guess if there is no mmu, there is no page fault, so no speculative page
fault and this patch is clearly required by the speculative page fault handler.
By the I should probably make CONFIG_SPECULATIVE_PAGE_FAULT dependent on
CONFIG_MMU.

This being said, if your idea is to extend the mm_rb tree rwlocking to the
nommu case, then this is another story, and I wondering if there is a real need
in such case. But I've to admit I'm not so familliar with kernel built for
mmuless systems.

Am I missing something ?

Thanks,
Laurent.