On Tue, Jan 14, 2020 at 1:28 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > On Tue, Jan 14, 2020 at 12:39:00PM -0800, Dan Williams wrote: > > On Tue, Jan 14, 2020 at 12:31 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > > > > > On Thu, Jan 09, 2020 at 12:03:01PM -0800, Dan Williams wrote: > > > > On Thu, Jan 9, 2020 at 3:27 AM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > > > On Tue 07-01-20 10:49:55, Dan Williams wrote: > > > > > > On Tue, Jan 7, 2020 at 10:33 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > > > > > > W.r.t partitioning, bdev_dax_pgoff() seems to be the pain point where > > > > > > > dax code refers back to block device to figure out partition offset in > > > > > > > dax device. If we create a dax object corresponding to "struct block_device" > > > > > > > and store sector offset in that, then we could pass that object to dax > > > > > > > code and not worry about referring back to bdev. I have written some > > > > > > > proof of concept code and called that object "dax_handle". I can post > > > > > > > that code if there is interest. > > > > > > > > > > > > I don't think it's worth it in the end especially considering > > > > > > filesystems are looking to operate on /dev/dax devices directly and > > > > > > remove block entanglements entirely. > > > > > > > > > > > > > IMHO, it feels useful to be able to partition and use a dax capable > > > > > > > block device in same way as non-dax block device. It will be really > > > > > > > odd to think that if filesystem is on /dev/pmem0p1, then dax can't > > > > > > > be enabled but if filesystem is on /dev/mapper/pmem0p1, then dax > > > > > > > will work. > > > > > > > > > > > > That can already happen today. If you do not properly align the > > > > > > partition then dax operations will be disabled. This proposal just > > > > > > extends that existing failure domain to make all partitions fail to > > > > > > support dax. > > > > > > > > > > Well, I have some sympathy with the sysadmin that has /dev/pmem0 device, > > > > > decides to create partitions on it for whatever (possibly misguided) > > > > > reason and then ponders why the hell DAX is not working? And PAGE_SIZE > > > > > partition alignment is so obvious and widespread that I don't count it as a > > > > > realistic error case sysadmins would be pondering about currently. > > > > > > > > > > So I'd find two options reasonably consistent: > > > > > 1) Keep status quo where partitions are created and support DAX. > > > > > 2) Stop partition creation altogether, if anyones wants to split pmem > > > > > device further, he can use dm-linear for that (i.e., kpartx). > > > > > > > > > > But I'm not sure if the ship hasn't already sailed for option 2) to be > > > > > feasible without angry users and Linus reverting the change. > > > > > > > > Christoph? I feel myself leaning more and more to the "keep pmem > > > > partitions" camp. > > > > > > > > I don't see "drop partition support" effort ending well given the long > > > > standing "ext4 fails to mount when dax is not available" precedent. > > > > > > > > I think the next least bad option is to have a dax_get_by_host() > > > > variant that passes an offset and length pair rather than requiring a > > > > later bdev_dax_pgoff() to recall the offset. This also prevents > > > > needing to add another dax-device object representation. > > > > > > I am wondering what's the conclusion on this. I want to this to make > > > progress in some direction so that I can make progress on virtiofs DAX > > > support. > > > > I think we should at least try to delete the partition support and see > > if anyone screams. Have a module option to revert the behavior so > > people are not stuck waiting for the revert to land, but if it stays > > quiet then we're in a better place with that support pushed out of the > > dax core. > > Hi Dan, > > So basically keep partition support code just that disable it by default > and it is enabled by some knob say kernel command line option/module > option. Yes. > At what point of time will we remove that code completely. I mean what > if people scream after two kernel releases, after we have removed the > code. I'd follow the typical timelines of Documentation/ABI/obsolete which is a year or more. > > Also, from distribution's perspective, we might not hear from our > customers for a very long time (till we backport that code in to > existing releases or release this new code in next major release). From > that view point I will not like to break existing user visible behavior. > > How bad it is to keep partition support around. To me it feels reasonaly > simple where we just have to store offset into dax device into another > dax object: If we end up keeping partition support, we're not adding another object. > and pass that object around (instead of dax_device). If that's > the case, I am not sure why to even venture into a direction where some > user's setup might be broken. It was a mistake to support them. If that mistake can be undone without breaking existing deployments the code base is better off without the concept. > Also from an application perspective, /dev/pmem is a block device, so it > should behave like a block device, (including kernel partition table support). > From that view, dax looks like just an additional feature of that device > which can be enabled by passing option "-o dax". dax via block devices was a crutch that we leaned on too heavily, and the implementation has slowly been moving away from it ever since. > IOW, can we reconsider the idea of not supporting kernel partition tables > for dax capable block devices. I can only see downsides of removing kernel > partition table support and only upside seems to be little cleanup of dax > core code. Can you help find end users that depend on it? Even the Red Hat installation guide example shows mounting on pmem0 directly. [1] My primary concern is people that might be booting from pmem as boot support requires an EFI partition table, and initramfs images would need to be respun to move to kpartx. [1]: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html-single/storage_administration_guide/index#Configuring-Persistent-Memory-for-File-System-Direct-Access-DAX
