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

[Hugh Dickins <hughd@google.com>]

If Andrew is happy with such a long patch name, okay;
but personally I'd prefer brevity to all that detail:

mm: Extend MREMAP_DONTUNMAP to non-anonymous mappings

On Wed, 17 Mar 2021, 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 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@linux-foundation.org/
> [2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@google.com/
> 
> Signed-off-by: Brian Geffon <bgeffon@google.com>

Acked-by: Hugh Dickins <hughd@google.com>

Thanks Brian, just what I wanted :)

You wondered in another mail about this returning -EINVAL whereas
the VM_DONTEXPAND size error returns -EFAULT: I've pondered, and I've
read the manpage, and I'm sure it would be wrong to change the old
-EFAULT to -EINVAL now; and I don't see good reason to change your
-EINVAL to -EFAULT either.  Let them differ, that's okay (and it's
only in special corner cases that either of these fail anyway).

> ---
>  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