Hello everyone, On Wed, Feb 12, 2020 at 12:14:16PM -0500, Peter Xu wrote: > Right. AFAICT QEMU uses it far more than disk IOs. A guest page can > be accessed by any kernel component on the destination host during a > postcopy procedure. It can be as simple as when a vcpu writes to a > missing guest page which still resides on the source host, then KVM > will get a page fault and trap into userfaultfd asking for that page. > The same thing happens to other modules like vhost, etc., as long as a > missing guest page is touched by a kernel module. Correct. How does the android garbage collection work to make sure there cannot be kernel faults on the missing memory? If I understood correctly (I didn't have much time to review sorry) what's proposed with regard to limiting uffd events from kernel faults, the only use case I know that could deal with it is the UFFD_FEATURE_SIGBUS but that's not normal userfaultfd: that's also the only feature required from uffd to implement a pure malloc library in userland that never takes the mmap sem for writing to implement userland mremap/mmap/munmap lib calls (as those will convert to UFFDIO_ZEROPAGE and MADV_DONTNEED internally to the lib and there will be always a single vma). We just need to extend UFFDIO_ZEROPAGE to map the THP zeropage to make this future pure-uffd malloc lib perform better. On the other end I'm also planning a mremap_vma_merge userland syscall that will merge fragmented vmas. Currently once you have a nice heap all contiguous but with small objects and you free the fragments you can't build THP anymore even if you make the memory virtually contiguous again by calling mremap. That just build up a ton of vmas slowing down the app forever and also preventing THP collapsing ever again. mremap_vma_merge will require no new kernel feature, but it fundamentally must be able to handle kernel faults. If databases starts to use that, how can you enable this feature without breaking random apps then? So it'd be a feature usable only by one user (Android) perhaps? And only until you start defragging the vmas of small objects? Thanks, Andrea
