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

[David Rientjes]

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()?