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From:   Brian Geffon <bgeffon@google.com>
Date:   Wed, 3 Mar 2021 10:13:18 -0800
Message-ID: <CADyq12ymZcrU-+ZnDZ=nieq5zgzf_j9zGT7-BPDXXnxP-rz_kw@mail.gmail.com>
Subject: Re: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP
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>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <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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I apologize, this patch didn't include my signed off by, here it is:

Signed-off-by: Brian Geffon <bgeffon@google.com>


On Wed, Mar 3, 2021 at 9:53 AM Brian Geffon <bgeffon@google.com> wrote:
>
> 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
>