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

[Christoph Hellwig]

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.