Patch series "mm: Extend MREMAP_DONTUNMAP to non-anonymous mappings", v5.

This patch (of 3):

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.

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%25akpm@linux-foundation.org/
[2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@google.com/

Link: https://lkml.kernel.org/r/20210323182520.2712101-1-bgeffon@google.comSigned-off-by: Brian Geffon <bgeffon@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Tested-by: Lokesh Gidra <lokeshgidra@google.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Alejandro Colomar <alx.manpages@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: "Michael S . Tsirkin" <mst@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sonny Rao <sonnyrao@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>