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.
