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