linux-kernel - Re: [PATCH v2 2/3] mm, dax: add VM_DAX flag for DAX VMAs

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Message-ID: <20160915170942.GJ9314@birch.djwong.org>
Date:   Thu, 15 Sep 2016 10:09:42 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        XFS Developers <xfs@....sgi.com>,
        Linux MM <linux-mm@...ck.org>,
        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 10:01:03AM -0700, Dan Williams wrote:
> 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?

Seeing as FS_IOC_FSGETXATTR is a "generic" ioctl now, why not just
implement it for all the DAX fses and block devices?  Aside from xflags,
the other fields are probably all zero for non-xfs (aside from project
quota id I guess).

(Yeah, sort of awkward, I know...)

--D

> 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.
> 
> _______________________________________________
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> xfs@....sgi.com
> http://oss.sgi.com/mailman/listinfo/xfs

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