On Wed, 17 Mar 2021 14:41:46 -0700 Brian Geffon <bgef...@google.com> wrote:
> Currently MREMAP_DONTUNMAP only accepts private anonymous mappings. This > change will widen the support to include any mappings which are not > VM_DONTEXPAND or VM_PFNMAP. Please update changelog to explain why these two were omitted? > The primary use case is to support > MREMAP_DONTUNMAP on mappings which may have been created from a memfd. > > This change will result in mremap(MREMAP_DONTUNMAP) returning -EINVAL > if VM_DONTEXPAND or VM_PFNMAP mappings are specified. > > Lokesh Gidra who works on the Android JVM, provided an explanation of how > such a feature will improve Android JVM garbage collection: > "Android is developing a new garbage collector (GC), based on userfaultfd. > The garbage collector will use userfaultfd (uffd) on the java heap during > compaction. On accessing any uncompacted page, the application threads will > find it missing, at which point the thread will create the compacted page > and then use UFFDIO_COPY ioctl to get it mapped and then resume execution. > Before starting this compaction, in a stop-the-world pause the heap will be > mremap(MREMAP_DONTUNMAP) so that the java heap is ready to receive > UFFD_EVENT_PAGEFAULT events after resuming execution. > > To speedup mremap operations, pagetable movement was optimized by moving > PUD entries instead of PTE entries [1]. It was necessary as mremap of even > modest sized memory ranges also took several milliseconds, and stopping the > application for that long isn't acceptable in response-time sensitive > cases. > > With UFFDIO_CONTINUE feature [2], it will be even more efficient to > implement this GC, particularly the 'non-moveable' portions of the heap. > It will also help in reducing the need to copy (UFFDIO_COPY) the pages. > However, for this to work, the java heap has to be on a 'shared' vma. > Currently MREMAP_DONTUNMAP only supports private anonymous mappings, this > patch will enable using UFFDIO_CONTINUE for the new userfaultfd-based heap > compaction." Is a manpage update planned? It's appropriate to add this to the series so folks can check it over. Can we please get the appropriate updates into tools/testing/selftests/vm/mremap_test.c for this? > Signed-off-by: Brian Geffon <bgef...@google.com> v3 is getting up there. Has there been much review activity?
