On Thu, Jun 28, 2018 at 05:42:34PM +0000, Kani, Toshi wrote:
On Tue, 2018-06-26 at 16:04 -0600, Ross Zwisler wrote:
On Tue, Jun 26, 2018 at 02:51:52PM -0700, Dan Williams wrote:
On Tue, Jun 26, 2018 at 2:31 PM, Kani, Toshi toshi.kani@hpe.com wrote:
On Tue, 2018-06-26 at 14:28 -0700, Dan Williams wrote:
On Tue, Jun 26, 2018 at 2:23 PM, Kani, Toshi toshi.kani@hpe.com wrote:
On Tue, 2018-06-26 at 14:02 -0700, Dan Williams wrote: > On Tue, Jun 26, 2018 at 1:54 PM, Kani, Toshi toshi.kani@hpe.com wrote:
[..]
> > When this dm change was made, the pmem driver supported DAX for both raw > > and memory modes (note: sector mode does not use the pmem driver). I > > think the issue was introduced when we dropped DAX support from raw > > mode. > > Still DAX with raw mode never really worked any way. It was also > something that was broken from day one. So what happens to someone who > happened to avoid all the problems with page-less DAX and enabled > device-mapper on top? That failure mode detail needs to be added to > this changelog if we want to propose this for -stable.
My point is that the behavior should be consistent between pmem and device-mapper. When -o dax succeeds on a pmem, then it should succeed on a device-mapper on top of that pmem.
Has the drop of dax support from raw mode made to -stable back to the baseline accepted 545ed20e6df6? It will introduce inconsistency, otherwise.
That commit, 569d0365f571 "dax: require 'struct page' by default for filesystem dax", has not been tagged for -stable.
Then, Fixes tag should be set to 569d0365f571 to keep the behavior consistent.
Sure, and the failure mode is...? I'm thinking the commit log should say:
"Starting with commit 569d0365f571 "dax: require 'struct page' by default for filesystem dax", dax is no longer supported for page-less configurations. However, device-mapper sees the QUEUE_FLAG_DAX still being set and falsely assumes that DAX is enabled, this leads to <insert user visible failure mode details here>"
Dan is correct that there is no user visible change for this. It is the right thing to do for consistency and sanity, but it doesn't actually have user visible behavior that needs to be backported to stable.
Toshi is correct that this change is only for raw mode namespaces, not btt namespaces.
I'll adjust the changelog and remove the stable flag for v5, and I'll add a Fixes: tag for patch 2.
Hi Ross,
Your patches look good. But I am still not clear about the Fixes & stable handling. Talking about user visible behavior, I do not think we had any issue until dax support was dropped from raw mode. Until then, the pmem driver supported dax for all modes, and the check for direct_access worked.
I agree that the fsdax + raw mode failure mode I mentioned in my cover letter only started when we restricted filesystem DAX to having struct page, but I think that the other failure mode, fsdax + some random block driver (I used brd) was present in DM from the beginning.
In any case, I think both are fixed with the patches, and I think it's fine that all 3 get thrown at stable. Thanks, Mike, for the help.