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From:   Brian Geffon <bgeffon@google.com>
Date:   Wed, 17 Mar 2021 17:25:14 -0400
Message-ID: <CADyq12xdH=WQCLU=Y9fZQKBca9OAb-Yco64WcLtZ1vp95fNEPA@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP mappings
 with MREMAP_DONTUNMAP
To:     Peter Xu <peterx@redhat.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>
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You're 100% correct, I'll mail a new patch in a few

Brian


On Wed, Mar 17, 2021 at 5:19 PM Peter Xu <peterx@redhat.com> wrote:
>
> 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
>