Re: [Lsf-pc] [LSF/MM TOPIC] Future direction of DAX

From: Dan Williams <dan.j.williams@intel.com>
To: Jan Kara <jack@suse.cz>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-block@vger.kernel.org,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	lsf-pc@lists.linux-foundation.org, Linux MM <linux-mm@kvack.org>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] Future direction of DAX
Date: Tue, 17 Jan 2017 08:56:52 -0800	[thread overview]
Message-ID: <CAPcyv4hO5ZjrBk=L1DLkf4SP5fFeTAD+o7GUQDv0fcJj4Q+pCg@mail.gmail.com> (raw)
In-Reply-To: <20170117155910.GU2517@quack2.suse.cz>

On Tue, Jan 17, 2017 at 7:59 AM, Jan Kara <jack@suse.cz> wrote:
> On Fri 13-01-17 17:20:08, Ross Zwisler wrote:
>> - The DAX fsync/msync model was built for platforms that need to flush dirty
>>   processor cache lines in order to make data durable on NVDIMMs.  There exist
>>   platforms, however, that are set up so that the processor caches are
>>   effectively part of the ADR safe zone.  This means that dirty data can be
>>   assumed to be durable even in the processor cache, obviating the need to
>>   manually flush the cache during fsync/msync.  These platforms still need to
>>   call fsync/msync to ensure that filesystem metadata updates are properly
>>   written to media.  Our first idea on how to properly support these platforms
>>   would be for DAX to be made aware that in some cases doesn't need to keep
>>   metadata about dirty cache lines.  A similar issue exists for volatile uses
>>   of DAX such as with BRD or with PMEM and the memmap command line parameter,
>>   and we'd like a solution that covers them all.
>
> Well, we still need the radix tree entries for locking. And you still need
> to keep track of which file offsets are writeably mapped (which we
> currently implicitely keep via dirty radix tree entries) so that you can
> writeprotect them if needed (during filesystem freezing, for reflink, ...).
> So I think what is going to gain the most by far is simply to avoid doing
> the writeback at all in such situations.

I came to the same conclusion when taking a look at this. I have some
patches that simply make the writeback optional, but do not touch any
of the other dirty tracking infrastructure. I'll send them out shortly
after a bit more testing. This also dovetails with the request from
Linus to push pmem flushing routines into the driver and stop abusing
__copy_user_nocache.

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