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Date: Fri, 1 Sep 2017 00:34:02 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 2/2] mm: introduce MAP_VALIDATE, a mechanism for for
 safely defining new mmap flags
Message-ID: <20170901073402.GA19080@infradead.org>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>, Christoph Hellwig <hch@lst.de>, Linux MM <linux-mm@kvack.org>, Jan Kara <jack@suse.cz>, Arnd Bergmann <arnd@arndb.de>, "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, Linux API <linux-api@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>, Andrew Morton <akpm@linux-foundation.org>

On Thu, Aug 31, 2017 at 06:27:31PM -0700, Linus Torvalds wrote:
> Why? That's no different from the case statement for the mmu case,
> just written differently.

Yes.

> You *want* existing kernels to fail, since they don't test the bits
> you want to test.
> 
> So you just want to rewrite these all as
> 
>     switch (flags & MAP_TYPE) {
>     case MAP_SHARED_VALIDATE:
>         .. validate the other bits...
>         /* fallhtough */
>     case MAP_SHARED:
>         .. do the shared case ..
>     case MAP_PRIVATE:
>         .. do the private case ..
>     default:
>         return -EINVAL;
>     }

Btw, at least my original idea was to make MAP_VALIDATE a flag
instead of another mapping type, that is take it out of MAP_TYPE.

That being said this version is ok with me too - the chances of
needing a new type of private mappings probably isn't too big.

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