linux-kernel - Re: [PATCH v3 3/4] dm: add infrastructure for DAX support

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Date:	Fri, 24 Jun 2016 11:44:47 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Kani, Toshimitsu" <toshi.kani@....com>
Cc:	"axboe@...nel.dk" <axboe@...nel.dk>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"yigal@...xistor.com" <yigal@...xistor.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v3 3/4] dm: add infrastructure for DAX support

On Fri, Jun 24 2016 at 11:40am -0400,
Kani, Toshimitsu <toshi.kani@....com> wrote:

> On Thu, 2016-06-23 at 21:49 -0400, Mike Snitzer wrote:
> > On Thu, Jun 23 2016 at  7:36pm -0400,
> > Kani, Toshimitsu <toshi.kani@....com> wrote:
>  :
> > > Thanks for the update.  I have a question about the above change.  Targets
> > > may have their own parameters.  For instance, dm-stripe has 'chunk_size',
> > > which is checked in stripe_ctr().  DAX adds additional restriction that
> > > chunk_size needs to be aligned by page size.  So, I think we need to keep
> > > target responsible to verify if DAX can be supported.  What do you think?
> >
> > We've never had to concern the dm-stripe target with hardware
> > specific chunk_size validation.  The user is able to specify the
> > chunk_size via lvm2's lvcreate -I argument.  Yes this gives users enough
> > rope to hang themselves but it is very easy to configure a dm-stripe
> > device with the appropriate chunk size (PAGE_SIZE) from userspace.
> > 
> > But lvm2 could even be trained to make sure the chunk_size is a factor
> > of physical_block_size (PAGE_SIZE in the case of pmem) if the underlying
> > devices export queue/dax=1
> 
> lvcreate -I only allows multiple of page size, so we are OK with lvm2.  I was
> wondering if the check in lvm2 is enough.  Are there any other tools that may
> be used to configure stripe size?  Can we trust userspace on this?

Other than lvm2, I'm not aware of any other userspace tool that is
driving the dm-stripe target configuration.  So I think we can trust
userspace here until proven otherwise.  Good news is that any
misconfiguration will simply not work right?  Errors would result from
improperly sized IO right?