On 3/23/21 6:25 PM, Brian Geffon wrote:
> Currently MREMAP_DONTUNMAP only accepts private anonymous mappings.
> This restriction was placed initially for simplicity and not because
> there exists a technical reason to do so.
>
> This change will widen the support to include any mappings which are not
> VM_DONTEXPAND or VM_PFNMAP. 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.
Pretty interesting idea :-)
>
> 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."
>
> [1] https://lore.kernel.org/linux-mm/20201215030730.NC3CU98e4%[email protected]/
> [2] https://lore.kernel.org/linux-mm/[email protected]/
>
> Signed-off-by: Brian Geffon <[email protected]>
> Acked-by: Hugh Dickins <[email protected]>
> Tested-by: Lokesh Gidra <[email protected]>
Reviewed-by: Dmitry Safonov <[email protected]>
Thanks,
Dmitry