linux-kernel - Re: [RFC PATCH 1/2] mm, mincore2(): retrieve dax and tlb-size attributes of an address range

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Message-ID: <20160913064405.GA21069@infradead.org>
Date:   Mon, 12 Sep 2016 23:44:05 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, linux-nvdimm@...1.01.org,
        linux-api@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH 1/2] mm, mincore2(): retrieve dax and tlb-size
 attributes of an address range

On Sun, Sep 11, 2016 at 10:31:35AM -0700, Dan Williams wrote:
> As evidenced by this bug report [1], userspace libraries are interested
> in whether a mapping is DAX mapped, i.e. no intervening page cache.
> Rather than using the ambiguous VM_MIXEDMAP flag in smaps, provide an
> explicit "is dax" indication as a new flag in the page vector populated
> by mincore.

And how exactly does an implementation detail like DAX matter for an
application?  The only thing that might matter is the atomicy boundary,
but mincore is not the right interface for that.