Re: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP

[Lokesh Gidra <lokeshgidra@google.com>]

On Wed, Mar 3, 2021 at 10:13 AM Brian Geffon <bgeffon@google.com> wrote:
>
> 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/

Thanks for the patch, Brian. I've tested mremap(MREMAP_DONTUNMAP) on a
memfd memory range.

Tested-by: Lokesh Gidra <lokeshgidra@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
> >