[lttng-dev] Progress on system crash traces with LTTng using DAX and pmem

[Mathieu Desnoyers]

----- Original Message -----
> From: "Matthew Wilcox" <willy at linux.intel.com>
> To: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> Cc: "Matthew Wilcox" <willy at linux.intel.com>, "Ross Zwisler" <ross.zwisler at linux.intel.com>, "lttng-dev"
> <lttng-dev at lists.lttng.org>, linux-fsdevel at vger.kernel.org, linux-mm at kvack.org, linux-kernel at vger.kernel.org
> Sent: Monday, October 27, 2014 2:48:09 PM
> Subject: Re: Progress on system crash traces with LTTng using DAX and pmem
> 
> On Sat, Oct 25, 2014 at 12:51:25PM +0000, Mathieu Desnoyers wrote:
> > A quick follow up on my progress on using DAX and pmem with
> > LTTng. I've been able to successfully gather a user-space
> > trace into buffers mmap'd into an ext4 filesystem within
> > a pmem block device mounted with -o dax to bypass the page
> > cache. After a soft reboot, I'm able to mount the partition
> > again, and gather the very last data collected in the buffers
> > by the applications. I created a "lttng-crash" program that
> > extracts data from those buffers and converts the content
> > into a readable Common Trace Format trace. So I guess
> > you have a use-case for your patchsets on commodity hardware
> > right there. :)
> 
> Sweet!
> 
> > I've been asked by my customers if DAX would work well with
> > mtd-ram, which they are using. To you foresee any roadblock
> > with this approach ?
> 
> Looks like we'd need to add support to mtd-blkdevs.c for DAX.  I assume
> they're already using one of the block-based ways to expose MTD to
> filesystems, rather than jffs2/logfs/ubifs?
> 
> I'm thinking we might want to add a flag somewhere in the block_dev / bdi
> that indicates whether DAX is supported.  Currently we rely on whether
> ->direct_access is present in the block_device_operations to indicate
> that, so we'd have to have two block_dev_operations in mtd-blkdevs,
> depending on whether direct access is supported by the underlying
> MTD device.  Not a show-stopper.
> 
> > Please keep me in CC on your next patch versions. I'm willing
> > to spend some more time reviewing them if needed. By the way,
> > do you guys have a target time-frame/kernel version you aim
> > at for getting this work upstream ?
> 
> We're trying to get it upstream ASAP.  We've been working on it
> publically since December last year, and it's getting frustrating that
> it's not upstream already.  I sent a v12 a few minutes before you sent
> this message ...  I thought git would add you to the cc's since your
> Reviewed-by is on some of the patches.

Hi Matthew,

I've noticed that Andrew Morton picked up your DAX patchset, which is
really good news!

About the topic of DAX support on mtd-ram: I'm wonder if we would
need the pmem patchset at all if mtd-ram gets DAX support ? How
do the two approaches differ ? Has anyone tried out mtd-ram over
DAX at this point ?

Thanks for the great work! :)

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com