linux-ext4 - Re: Subtle races between DAX mmap fault and write path

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Message-ID: <86k2g15gh8.fsf@hiro.keithp.com>
Date:	Sun, 31 Jul 2016 20:13:23 -0700
From:	Keith Packard <keithp@...thp.com>
To:	Dave Chinner <david@...morbit.com>,
	Dan Williams <dan.j.williams@...el.com>
Cc:	Jan Kara <jack@...e.cz>,
	"linux-nvdimm\@lists.01.org" <linux-nvdimm@...ts.01.org>,
	XFS Developers <xfs@....sgi.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: Subtle races between DAX mmap fault and write path

Dave Chinner <david@...morbit.com> writes:

> So we'd see that from the point of view of a torn single sector
> write. Ok, so we better limit DAX to CRC enabled filesystems to
> ensure these sorts of events are always caught by the filesystem.

Which is the same lack of guarantee that we already get on rotating
media. Flash media seems to work harder to provide sector atomicity; I
guess that's a feature?

The alternative is to hide metadata behind a translation layer, and
while that can be done for lame file systems, I'd like to see the raw
hardware capabilities exposed and then make free software that
constructs a reliable system on top of that.

-- 
-keith

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