linux-kernel - Re: [PATCH v2 7/7] pmem, dax: have direct_access use _

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Message-ID: <1439842038.11296.1.camel@linux.intel.com>
Date:	Mon, 17 Aug 2015 14:07:18 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Jonathan Corbet <corbet@....net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Jens Axboe <axboe@...nel.dk>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux390@...ibm.com, Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Jeff Layton <jlayton@...marydata.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Omar Sandoval <osandov@...ndov.com>,
	Boaz Harrosh <boaz@...xistor.com>,
	Miklos Szeredi <mszeredi@...e.cz>, Jan Kara <jack@...e.cz>,
	Wolfram Sang <wsa@...-dreams.de>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	Ameen Ali <ameenali023@...il.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Sagi Grimberg <sagig@...lanox.com>,
	Mike Snitzer <snitzer@...hat.com>, Tejun Heo <tj@...nel.org>,
	Shaohua Li <shli@...com>, Ming Lei <ming.lei@...onical.com>,
	linux-doc@...r.kernel.org,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	linux-s390@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v2 7/7] pmem, dax: have direct_access use __pmem
 annotation

On Sat, 2015-08-15 at 08:44 -0700, Dan Williams wrote:
> On Sat, Aug 15, 2015 at 2:19 AM, Christoph Hellwig <hch@....de> wrote:
> > On Thu, Aug 13, 2015 at 10:51:11AM -0600, Ross Zwisler wrote:
> >> Update the annotation for the kaddr pointer returned by direct_access()
> >> so that it is a __pmem pointer.  This is consistent with the PMEM driver
> >> and with how this direct_access() pointer is used in the DAX code.
> >
> > IFF we stick to the __pmem annotations this looks good.
> >
> > That beeing said I start to really dislike them.  We don't special
> > accesors to read/write from pmem, we just need to explicitly commit
> > it if we want to make it persistent.  So I really don't see the need
> > to treat it special and require all the force casts to and from the
> > attribute.
> 
> I'm not going to put up much of a fight if it's really getting in the way....
> 
> That said, while we don't need special accessors we do need guarantees
> that anything that has written to a persistent memory address has done
> so in a way that wmb_pmem() is able to flush it.  It's more of a "I've
> audited this code path for wmb_pmem() compatibility so use this api to
> write to pmem."
> 
> Perhaps a better way to statically check for missed flushes might be
> to have acquire_pmem_for_write() + release() annotations and the final
> release does a wmb_pmem(), but as far as I can tell the sparse
> acquire/release annotations don't stack.

FWIW I've been on the fence about the __pmem annotations, but my current
thought is that we really do need a way of saying that stores to these
pointers need special care for wmb_pmem() to do its thing and that __pmem does
a reasonably good job of that.  If we can figure out a cooler way, such as the
write() + release() flow Dan is talking about, great.  But I think we need
something to keep us from making errors by storing to PMEM pointers and
leaving data in the processor cache.


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