From: Ross Zwisler <email@example.com> To: Mike Snitzer <firstname.lastname@example.org> Cc: Ross Zwisler <email@example.com>, Toshi Kani <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH v2 3/7] dm: fix test for DAX device support Date: Fri, 1 Jun 2018 15:11:13 -0600 [thread overview] Message-ID: <20180601211113.GB12821@linux.intel.com> (raw) In-Reply-To: <20180601204604.GB1144@redhat.com> On Fri, Jun 01, 2018 at 04:46:04PM -0400, Mike Snitzer wrote: > On Fri, Jun 01 2018 at 4:19P -0400, > Mike Snitzer <firstname.lastname@example.org> wrote: > > > On Tue, May 29 2018 at 3:51P -0400, > > Ross Zwisler <email@example.com> wrote: > > > > > Currently device_supports_dax() just checks to see if the QUEUE_FLAG_DAX > > > flag is set on the device's request queue to decide whether or not the > > > device supports filesystem DAX. This is insufficient because there are > > > devices like PMEM namespaces in raw mode which have QUEUE_FLAG_DAX set but > > > which don't actually support DAX. > > > > Isn't that a PMEM bug then? > > > > What is the point of setting QUEUE_FLAG_DAX if it cannot be trusted? Yup, that seems like a bug. > > > This means that you could create a dm-linear device, for example, where the > > > first part of the dm-linear device was a PMEM namespace in fsdax mode and > > > the second part was a PMEM namespace in raw mode. Both DM and the > > > filesystem you put on that dm-linear device would think the whole device > > > supports DAX, which would lead to bad behavior once your raw PMEM namespace > > > part using DAX needed struct page for something. > > > > The PMEM namespace in raw mode shouldn't be setting QUEUE_FLAG_DAX, if > > it didn't then the stacked-up linear DM wouldn't > > > > > Fix this by using bdev_dax_supported() like filesystems do at mount time. > > > This checks for raw mode and also performs other tests like checking to > > > make sure the dax_direct_access() path works. > > > > Sorry "This" does those things where? > > I see you meant bdev_dax_supported() does these additional checks. > > My previous question stands though. Why is QUEUE_FLAG_DAX getting set > if the device hasn't already passed these checks? Shouldn't setting > QUEUE_FLAG_DAX on request_queue depend on bdev_dax_supported() passing? > > But looking at the drivers that do set QUEUE_FLAG_DAX: they > don't have the bdev readily available. Anyway, just strikes me as > bizarre that a driver can set QUEUE_FLAG_DAX without having to have > ensured bdev_dax_supported() passes (even if not programatically, but > that the developer has verified the hooks, et al exist). I agree that it's a bug that QUEUE_FLAG_DAX is set for raw mode namespaces, thanks for pointing that out. I will fix that, but I still think that DM should be using bdev_dax_supported() like the filesystems do. Even beyond the raw mode / fsdax mode thing, this does additional checks that are useful. So, I think these DM patches are still correct. > But I'll give up on this line of questioning.. > > My dilemma now is: how do I take these changes without first rebasing > linux-dm.git ontop of Darrick's xfs tree? > > I probably should've reviewed faster and been the one to take the entire > set (with appropriate acks obviously).
next prev parent reply other threads:[~2018-06-01 21:11 UTC|newest] Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-05-29 19:50 [PATCH v2 0/7] Fix DM DAX handling Ross Zwisler 2018-05-29 19:51 ` [PATCH v2 1/7] fs: allow per-device dax status checking for filesystems Ross Zwisler 2018-05-29 19:51 ` [PATCH v2 2/7] dax: change bdev_dax_supported() to support boolean returns Ross Zwisler 2018-05-29 21:25 ` Darrick J. Wong 2018-05-29 22:01 ` Ross Zwisler 2018-05-31 19:13 ` Darrick J. Wong 2018-05-31 20:34 ` Ross Zwisler 2018-05-31 20:35 ` Dan Williams 2018-05-31 20:41 ` Ross Zwisler 2018-05-31 20:52 ` Mike Snitzer 2018-05-31 22:26 ` [dm-devel] " Darrick J. Wong 2018-06-01 20:59 ` Ross Zwisler 2018-06-01 1:26 ` Dave Chinner 2018-06-01 1:57 ` Dan Williams 2018-06-01 2:24 ` Dave Chinner 2018-06-01 4:02 ` Dan Williams 2018-06-03 22:20 ` Dave Chinner 2018-06-04 0:25 ` Dave Chinner 2018-06-04 1:48 ` Dan Williams 2018-06-04 23:40 ` Dan Williams 2018-06-05 0:33 ` Mike Snitzer 2018-06-05 5:55 ` Dave Chinner 2018-06-05 3:32 ` Dan Williams 2018-05-29 19:51 ` [PATCH v2 3/7] dm: fix test for DAX device support Ross Zwisler 2018-06-01 20:19 ` Mike Snitzer 2018-06-01 20:46 ` Mike Snitzer 2018-06-01 21:11 ` Ross Zwisler [this message] 2018-06-01 21:16 ` Dan Williams 2018-05-29 19:51 ` [PATCH v2 4/7] dm: prevent DAX mounts if not supported Ross Zwisler 2018-06-01 21:55 ` Mike Snitzer 2018-06-04 23:15 ` Ross Zwisler 2018-06-20 15:17 ` Mike Snitzer 2018-06-25 19:20 ` Ross Zwisler 2018-05-29 19:51 ` [PATCH v2 5/7] dm: remove DM_TYPE_DAX_BIO_BASED dm_queue_mode Ross Zwisler 2018-06-01 22:04 ` Mike Snitzer 2018-06-04 23:24 ` Ross Zwisler 2018-06-04 23:49 ` Kani, Toshi 2018-06-05 0:46 ` Mike Snitzer 2018-06-06 17:24 ` Ross Zwisler 2018-06-06 22:29 ` Mike Snitzer 2018-05-29 19:51 ` [PATCH v2 6/7] dm-snap: remove unnecessary direct_access() stub Ross Zwisler 2018-05-29 19:51 ` [PATCH v2 7/7] dm-error: " Ross Zwisler
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