lkml.org 
[lkml]   [2018]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue_mode
Date
On Mon, 2018-06-04 at 17:24 -0600, Ross Zwisler wrote:
> On Fri, Jun 01, 2018 at 06:04:43PM -0400, Mike Snitzer wrote:
> > On Tue, May 29 2018 at 3:51pm -0400,
> > Ross Zwisler <ross.zwisler@linux.intel.com> wrote:
:
> > For example, the following transition will currently fail:
> >
> > dm-linear: [fsdax pmem][fsdax pmem] => [fsdax pmem][fsdax raw]
> > DM_TYPE_DAX_BIO_BASED DM_TYPE_BIO_BASED
> >
> > but these will both succeed:
> >
> > dm-linear: [fsdax pmem][brd ramdisk] => [fsdax pmem][fsdax raw]
> > DM_TYPE_BIO_BASED DM_TYPE_BIO_BASED
> >
> > dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem]
> > DM_TYPE_BIO_BASED DM_TYPE_DAX_BIO_BASED
>
> So we allow 2 of the 3 transitions, but the reason that we disallow the third
> isn't fully clear to me.

I need to refresh my memory for the code, but here is the intent.
https://lkml.org/lkml/2016/6/22/1000
https://lkml.org/lkml/2016/6/22/999


> > > dm-linear: [fsdax pmem][fsdax raw] => [fsdax pmem][fsdax pmem]
> > > DM_TYPE_BIO_BASED DM_TYPE_DAX_BIO_BASED
> > >
> > > This seems arbitrary, as really the choice on whether to use DAX happens at
> > > filesystem mount time. There's no guarantee that the in the first case
> > > (double fsdax pmem) we were using the dax mount option with our file
> > > system.
> > >
> > > Instead, get rid of DM_TYPE_DAX_BIO_BASED and all the special casing around
> > > it, and instead make the request queue's QUEUE_FLAG_DAX be our one source
> > > of truth. If this is set, we can use DAX, and if not, not. We keep this
> > > up to date in table_load() as the table changes. As with regular block
> > > devices the filesystem will then know at mount time whether DAX is a
> > > supported mount option or not.
> >
> > If you don't think you need this specialization that is fine.. but DM
> > devices supporting suspending (as part of table reloads) so is there any
> > risk that there will be inflight IO (say if someone did 'dmsetup suspend
> > --noflush').. and then upon reload the device type changed out from
> > under us.. anyway, I don't have all the PMEM DAX stuff paged back into
> > my head yet.
> >
> > But this just seems like we really shouldn't be allowing the
> > transition from what was DM_TYPE_DAX_BIO_BASED back to DM_TYPE_BIO_BASED
>
> I admit I don't fully understand all the ways that DM supports suspending and
> resuming devices. Is there actually a case where we can change out the DM
> devices while I/O is running, and somehow end up trying to issue a DAX I/O to
> a device that doesn't support DAX?
>
> Toshi, do you have a test case that shows this somehow?

No, I did not test suspend/resume since HPE servers do not support it.

Thanks,
-Toshi
\
 
 \ /
  Last update: 2018-06-05 01:49    [W:0.052 / U:38.440 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site