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

[Matthew Wilcox]

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.