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

[Brian Geffon]

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. 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@xxxxxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@xxxxxxxxxx/

Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx>
---
 mm/mremap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index ec8f840399ed..db5b8b28c2dd 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -653,8 +653,8 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 		return ERR_PTR(-EINVAL);
 	}
 
-	if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
-			vma->vm_flags & VM_SHARED))
+	if ((flags & MREMAP_DONTUNMAP) &&
+			(vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)))
 		return ERR_PTR(-EINVAL);
 
 	if (is_vm_hugetlb_page(vma))
-- 
2.31.0.rc2.261.g7f71774620-goog