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Date:   Wed, 17 Mar 2021 17:18:57 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Brian Geffon <bgeffon@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Hugh Dickins <hughd@google.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Dmitry Safonov <dima@arista.com>
Subject: Re: [PATCH v2 1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP
 mappings with MREMAP_DONTUNMAP
Message-ID: <20210317211857.GN395976@xz-x1>
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 <20210317204015.GM395976@xz-x1>
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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