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

From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>, Lokesh Gidra <lokeshgidra@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Brian Geffon <bgeffon@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Kalesh Singh <kaleshsingh@google.com>,
	Nicolas Geoffray <ngeoffray@google.com>,
	Jared Duke <jdduke@google.com>,
	android-mm <android-mm@google.com>,
	Blake Caldwell <blake.caldwell@colorado.edu>,
	Mike Rapoport <rppt@kernel.org>
Subject: Re: RFC for new feature to move pages from one vma to another without split
Date: Wed, 12 Apr 2023 10:47:52 +0200	[thread overview]
Message-ID: <27ac2f51-e2bf-7645-7a76-0684248a5902@redhat.com> (raw)
In-Reply-To: <ZC8BgFSFC3cDcAcS@x1n>

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