Re: [PATCH v2 1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP mappings with MREMAP_DONTUNMAP

[Peter Xu <peterx@redhat.com>]

Hi, Brian,

On Wed, Mar 17, 2021 at 12:13:33PM -0700, Brian Geffon wrote:
> 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 which takes advantage of the existing check in vma_to_resize
> for non-VM_DONTEXPAND and non-VM_PFNMAP mappings will cause
> MREMAP_DONTUNMAP to return -EFAULT if such mappings are remapped. This
> behavior is consistent with existing behavior when using mremap with
> such mappings.
> 
> 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/
> 
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> ---
>  mm/mremap.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index ec8f840399ed..2c57dc4bc8b6 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -653,10 +653,6 @@ 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))
> -		return ERR_PTR(-EINVAL);
> -
>  	if (is_vm_hugetlb_page(vma))
>  		return ERR_PTR(-EINVAL);

The code change seems to be not aligned with what the commit message said.  Did
you perhaps forget to add the checks against VM_DONTEXPAND | VM_PFNMAP?  I'm
guessing that (instead of commit message to be touched up) because you still
attached the revert patch, then that check seems to be needed.  Thanks,

-- 
Peter Xu