Re: Synchronization around mmap_changing in userfaultfd

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Thu, Jan 11, 2024 at 03:32:20PM -0800, Lokesh Gidra wrote:
> Hi,
> 
> We have been seeing mmap_lock contention issues while using
> userfaultfd for GC in Android. But now that per-vma locks are being
> used in the kernel, we were hoping to use it in userfaultfd code to
> pin the VMA in COPY/MOVE/ZEROPAGE etc. operations. But while going
> through the code, I noticed that mmap_changing is implicitly protected
> by mmap_lock:
> 
> 1) All increments to it (except for userfault_remove) are done with
> mmap_lock in write-mode
> 2) All reads (in copy/move/zeropage etc) are done with mmap_lock in read-mode
> 
> I wanted to understand if that's just out of convenience, and
> therefore would it be ok to introduce a read-write semaphore in
> userfaultfd_ctx to achieve the same synchronization:
> 
> 1) All increments are done with this semaphore in write-mode
> 2) All operations (copy/move/zeropage etc) are done within the
> critical section of this semaphore in read-mode and checking that
> mmap_changing is 0.

mmap_changing was added to the existing critical sections that were already
protected with mmap_lock, so I didn't see a reason for additional lock to
protect mmap_changing.
 
With per-vma locks, your proposal makes perfect sense to me.

> If this is wrong, then kindly explain why mmap_changing needs to be
> protected with mmap_lock.
> 
> 
> Thanks,
> Lokesh
> 

-- 
Sincerely yours,
Mike.