Re: [PATCH 3/7] xfs: protect S_DAX transitions in XFS read path

[Dan Williams]

On Tue, Sep 26, 2017 at 7:33 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> On Tue, Sep 26, 2017 at 06:59:37AM -0700, Dan Williams wrote:
>> > I think you probably want an IOCB_DAX flag to check IS_DAX once and
>> > then stick to it, similar to what we do for direct I/O.
>>
>> I wonder if this works better with a reference count mechanism
>> per-file so that we don't need a hold a lock over the whole
>> transition. Similar to request_queue reference counting, when DAX is
>> being turned off we block new references and drain the in-flight ones.
>
> Maybe.  But that assumes we want to be stuck in a perpetual binary
> DAX on/off state on a given file.  Which makes not only for an awkward
> interface (inode or mount flag), but also might be fundamentally the
> wrong thing to do for some media where you'd happily read directly
> from it but rather buffer writes in DRAM.

I think we'll always need an explicit override available, but yes we
need to think about what the override looks like in the context of a
kernel that is able to automatically pick the right I/O policy
relative to the media type. A potential mixed policy for reads vs
writes makes sense. Where would this finer grained I/O policy
selection go other than more inode flags?