Re: RFC for new feature to move pages from one vma to another without split

[David Hildenbrand]

On 06.04.23 19:29, Peter Xu wrote:
> Hi, Lokesh,
>
> Sorry for a late reply.  Copy Blake Caldwell and Mike too.
>
> On Thu, Feb 16, 2023 at 02:27:11PM -0800, Lokesh Gidra wrote:
> > I) SUMMARY:
> > Requesting comments on a new feature which remaps pages from one
> > private anonymous mapping to another, without altering the vmas
> > involved. Two alternatives exist but both have drawbacks:
> > 1. userfaultfd ioctls allocate new pages, copy data and free the old
> > ones even when updates could be done in-place;
> > 2. mremap results in vma splitting in most of the cases due to 'pgoff' mismatch.
>
> Personally it was always a mistery to me on how vm_pgoff works with
> anonymous vmas and why it needs to be setup with vm_start >> PAGE_SHIFT.
>
> Just now I tried to apply below oneliner change:
>
> @@ -1369,7 +1369,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>                          /*
>                           * Set pgoff according to addr for anon_vma.
>                           */
> -                       pgoff = addr >> PAGE_SHIFT;
> +                       pgoff = 0;
>                          break;
>                  default:
>                          return -EINVAL;
>
> The kernel even boots without a major problem so far..

I think it's for RMAP purposes.

Take a look at linear_page_index() and how it's, for example, used in 
ksm_might_need_to_copy() alongside page->index.

--
Thanks,

David / dhildenb