Re: [PATCH v3 1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP mappings with MREMAP_DONTUNMAP

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 17 Mar 2021 14:41:46 -0700 Brian Geffon <bgeffon@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 <bgeffon@google.com>

v3 is getting up there.  Has there been much review activity?