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Date:   Wed,  3 Mar 2021 09:52:35 -0800
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Subject: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP
From:   Brian Geffon <bgeffon@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Axel Rasmussen <axelrasmussen@google.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Brian Geffon <bgeffon@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andy Lutomirski <luto@amacapital.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>
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Precedence: bulk

Currently MREMAP_DONTUNMAP only accepts private anonymous mappings. This change
will widen the support to include shmem mappings. The primary use case
is to support MREMAP_DONTUNMAP on mappings which may have been created from
a memfd.

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/
---
 mm/mremap.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index ec8f840399ed..6934d199da54 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -653,8 +653,7 @@ 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_is_anonymous(vma) || vma_is_shmem(vma)))
 		return ERR_PTR(-EINVAL);
 
 	if (is_vm_hugetlb_page(vma))
-- 
2.31.0.rc0.254.gbdcc3b1a9d-goog