linux-kernel - Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

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Message-ID: <20160207083814.GA695@infradead.org>
Date:	Sun, 7 Feb 2016 00:38:14 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Jan Kara <jack@...e.cz>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>
Subject: Re: [PATCH 2/2] dax: fix bdev NULL pointer dereferences

On Fri, Feb 05, 2016 at 03:25:00PM -0700, Ross Zwisler wrote:
> 	According to the standard specification (e.g., POSIX.1-2001), sync()
> 	schedules the writes,  but  may  return before  the  actual  writing
> 	is done.  However, since version 1.3.20 Linux does actually wait.
> 	(This still does not guarantee data integrity: modern disks have large
> 	caches.) 
> 
> Based on this I don't believe that it is a requirement that sync and syncfs
> actually flush the data durably to media before they return - they just need
> to make sure it has been sent to the device, which is always true for all
> writes PMEM via DAX, even without any calls to dax_writeback_mapping_range().

For Linux there is a requirement to not return before the data is on disk,
and our users rely on it.  The man page is rather confusing, and I'll see
if I can fix it up.