Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

From: Verma, Vishal L
Date: Thu May 05 2016 - 17:45:17 EST

On Thu, 2016-05-05 at 08:22 -0700, Christoph Hellwig wrote:
> On Thu, May 05, 2016 at 08:15:32AM -0700, Dan Williams wrote:
> >
> > >
> > > Agreed - makig O_DIRECT less direct than not having it is plain
> > > stupid,
> > > and I somehow missed this initially.
> > Of course I disagree because like Dave argues in the msync case we
> > should do the correct thing first and make it fast later, but also
> > like Dave this arguing in circles is getting tiresome.
> We should do the right thing first, and make it fast later.ÂÂBut this
> proposal is not getting it right - it still does not handle errors
> for the fast path, but magically makes it work for direct I/O by
> in general using a less optional path for O_DIRECT.ÂÂIt's getting the
> worst of all choices.
> As far as I can tell the only sensible option is to:
> Â- always try dax-like I/O first
> Â- have a custom get_user_pages + rw_bytes fallback handles bad blocks
> ÂÂÂwhen hitting EIO

I'm not sure I completely understand how this will work? Can you explain
a bit? Would we have to export rw_bytes up to layers above the pmem
driver? Where does get_user_pages come in?

> And then we need to sort out the concurrent write synchronization.
> Again there I think we absolutely have to obey Posix for the !O_DIRECT
> case and can avoid it for O_DIRECT, similar to the existing non-DAX
> semantics.ÂÂIf we want any special additional semantics we _will_ need
> a special O_DAX flag.
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