linux-kernel - Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags

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Date:   Mon, 18 Sep 2017 08:47:59 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        david <david@...morbit.com>
Subject: Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding
 new mmap flags

On Mon, Sep 18, 2017 at 2:31 AM, Jan Kara <jack@...e.cz> wrote:
> On Sun 17-09-17 19:39:45, Christoph Hellwig wrote:
>> On Sat, Sep 16, 2017 at 08:44:14PM -0700, Dan Williams wrote:
>> > So it wasn't all that easy, and Linus declined to take it. I think we
>> > should add a new ->mmap_validate() file operation and save the
>> > tree-wide cleanup until later.
>>
>> Note that we already have a mmap_capabilities callout for nommu,
>> I wonder if we could generalize that.
>
> So if I understood Dan right, Linus refused to merge the patch which adds
> 'flags' argument to ->mmap callback. That is actually logically independent
> change from validating flags passed to mmap(2) syscall. Dan did it just to
> save himself from adding a VMA flag for MAP_DIRECT.
>
> For validating flags passed to mmap(2), I agree we could use
> ->mmap_capabilities() instead of mmap_supported_mask Dan has added. But I
> don't have a strong opinion there.

The drawback I see with mmap_capabilities is that it requires all mmap
flags to have a corresponding vm_flag. After the cold reaction the
VM_DAX flag received I'd want to be sure they were on board with this
direction.