linux-kernel - Re: [PATCH v2 2/3] mm, dax: add VM

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Date:   Thu, 15 Sep 2016 10:01:03 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Linux MM <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        david <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        XFS Developers <xfs@....sgi.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs

On Thu, Sep 15, 2016 at 1:26 AM, Christoph Hellwig <hch@....de> wrote:
> On Wed, Sep 14, 2016 at 11:54:38PM -0700, Dan Williams wrote:
>> The DAX property, page cache bypass, of a VMA is only detectable via the
>> vma_is_dax() helper to check the S_DAX inode flag.  However, this is
>> only available internal to the kernel and is a property that userspace
>> applications would like to interrogate.
>
> They have absolutely no business knowing such an implementation detail.

Hasn't that train already left the station with FS_XFLAG_DAX?

The other problem with hiding the DAX property is that it turns out to
not be a transparent acceleration feature.  See xfs/086 xfs/088
xfs/089 xfs/091 which fail with DAX and, as far as I understand, it is
due to the fact that DAX disallows delayed allocation behavior.

If behavior changes I think we should indicate that to userspace and
VM_DAX is certainly more useful to userspace than some of the other vm
internals we already export in those flags.