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. Thanks Vivek
