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

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Date:   Wed, 17 Mar 2021 17:18:57 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH v2 1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP
 mappings with MREMAP_DONTUNMAP

On Wed, Mar 17, 2021 at 04:44:25PM -0400, Brian Geffon wrote:
> Hi Peter,

Hi, Brian,

> Thank you as always for taking a look. This change relies on the
> existing check in vma_to_resize on line 686:
> https://elixir.bootlin.com/linux/v5.12-rc3/source/mm/mremap.c#L686
> which returns -EFAULT when the vma is VM_DONTEXPAND or VM_PFNMAP.

Do you mean line 676?

https://elixir.bootlin.com/linux/v5.12-rc3/source/mm/mremap.c#L676

I'm not sure whether it'll work for MREMAP_DONTUNMAP, since IIUC
MREMAP_DONTUNMAP only works for the remap case with no size change, however in
that case in vma_to_resize() we'll bail out even earlier than line 676 when
checking against the size:

https://elixir.bootlin.com/linux/v5.12-rc3/source/mm/mremap.c#L667

So IIUC we'll still need the change as Hugh suggested previously.

Thanks,

-- 
Peter Xu