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

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Date:   Wed, 12 Apr 2023 10:47:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, Lokesh Gidra <lokeshgidra@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Nicolas Geoffray <ngeoffray@...gle.com>,
        Jared Duke <jdduke@...gle.com>,
        android-mm <android-mm@...gle.com>,
        Blake Caldwell <blake.caldwell@...orado.edu>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: RFC for new feature to move pages from one vma to another without
 split

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