Re: [PATCH v3 1/3] pmem: only set QUEUE_FLAG_DAX for fsdax mode
From: Dan Williams <hidden>
Date: 2018-06-26 19:19:03
Also in:
dm-devel, linux-fsdevel, linux-xfs, lkml, nvdimm
On Tue, Jun 26, 2018 at 12:13 PM, Mike Snitzer [off-list ref] wrote:
On Tue, Jun 26 2018 at 3:07pm -0400, Dan Williams [off-list ref] wrote:quoted
On Tue, Jun 26, 2018 at 11:58 AM, Mike Snitzer [off-list ref] wrote:quoted
On Tue, Jun 26 2018 at 2:52pm -0400, Dan Williams [off-list ref] wrote:quoted
On Tue, Jun 26, 2018 at 10:59 AM, Ross Zwisler [off-list ref] wrote:quoted
QUEUE_FLAG_DAX is an indication that a given block device supports filesystem DAX and should not be set for PMEM namespaces which are in "raw" or "sector" modes. These namespaces lack struct page and are prevented from participating in filesystem DAX. Signed-off-by: Ross Zwisler <redacted> Suggested-by: Mike Snitzer <redacted> Cc: stable@vger.kernel.orgWhy is this cc: stable? What is the user visible impact of this change especially given the requirement to validate QUEUE_FLAG_DAX with bdev_dax_supported()? Patch looks good, but it's just a cosmetic fixup afaics.This isn't cosmetic when you consider that stacking up a DM device is looking at this flag to determine whether a table does or does _not_ support DAX. So this patch, in conjunction with the other changes in the series, is certainly something I'd consider appropriate for stable.I think this classifies as something that never worked correctly and is not a regression. It does not identify which commit it is repairing or the user visible failure mode.So you're taking issue with making stacked dax configs work in older kernels? That's fine. We can drop the stable cc if you like. But I mean we intended for this to work.. so the Fixes commit references can easily be added, e.g.: 545ed20e6df68a4d2584a29a2a28ee8b2f7e9547 ("dm: add infrastructure for DAX support")
...but to be clear stacked DAX configs are not broken, right? They just happen not to work in this corner case of pmem devices without pages. Given we dropped DAX support for page less devices what configurations are repaired by this change? Am I missing something? Is it a data corruption scenario if this DM gets the wrong idea about DAX support. I could be convinced to recommend this for -stable, but the changelog as written does not make a strong case.