On Wed, Aug 28, 2019 at 05:04:11PM -0700, Dan Williams wrote: > On Wed, Aug 28, 2019 at 3:53 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > > On Wed, Aug 28, 2019 at 01:58:43PM -0400, Vivek Goyal wrote: > > > On Tue, Aug 27, 2019 at 11:58:09PM -0700, Christoph Hellwig wrote: > > > > On Tue, Aug 27, 2019 at 12:38:28PM -0400, Vivek Goyal wrote: > > > > > > For bdev_dax_pgoff > > > > > > I'd much rather have the partition offset if there is on in the daxdev > > > > > > somehow so that we can get rid of the block device entirely. > > > > > > > > > > IIUC, there is one block_device per partition while there is only one > > > > > dax_device for the whole disk. So we can't directly move bdev logical > > > > > offset into dax_device. > > > > > > > > Well, then we need to find a way to get partitions for dax devices, > > > > as we really should not expect a block device hiding behind a dax > > > > dev. That is just a weird legacy assumption - block device need to > > > > layer on top of the dax device optionally. > > > > > > > > > > > > > > We probably could put this in "iomap" and leave it to filesystems to > > > > > report offset into dax_dev in iomap that way dax generic code does not > > > > > have to deal with it. But that probably will be a bigger change. > > > > > > > > And where would the file system get that information from? > > > > > > File system knows about block device, can it just call get_start_sect() > > > while filling iomap->addr. And this means we don't have to have > > > parition information in dax device. Will something like following work? > > > (Just a proof of concept patch). > > > > > > > > > --- > > > drivers/dax/super.c | 11 +++++++++++ > > > fs/dax.c | 6 +++--- > > > fs/ext4/inode.c | 6 +++++- > > > include/linux/dax.h | 1 + > > > 4 files changed, 20 insertions(+), 4 deletions(-) > > > > > > Index: rhvgoyal-linux/fs/ext4/inode.c > > > =================================================================== > > > --- rhvgoyal-linux.orig/fs/ext4/inode.c 2019-08-28 13:51:16.051937204 -0400 > > > +++ rhvgoyal-linux/fs/ext4/inode.c 2019-08-28 13:51:44.453937204 -0400 > > > @@ -3589,7 +3589,11 @@ retry: > > > WARN_ON_ONCE(1); > > > return -EIO; > > > } > > > - iomap->addr = (u64)map.m_pblk << blkbits; > > > + if (IS_DAX(inode)) > > > + iomap->addr = ((u64)map.m_pblk << blkbits) + > > > + (get_start_sect(iomap->bdev) * 512); > > > + else > > > + iomap->addr = (u64)map.m_pblk << blkbits; > > > > I'm not a fan of returning a physical device sector address from an > > interface where ever other user/caller expects this address to be a > > logical block address into the block device. It creates a landmine > > in the iomap API that callers may not be aware of and that's going > > to cause bugs. We're trying really hard to keep special case hacks > > like this out of the iomap infrastructure, so on those grounds alone > > I'd suggest this is a dead end approach. > > > > Hence I think that if the dax device needs a physical offset from > > the start of the block device the filesystem sits on, it should be > > set up at dax device instantiation time and so the filesystem/bdev > > never needs to be queried again for this information. > > > > Agree. In retrospect it was my laziness in the dax-device > implementation to expect the block-device to be available. > > It looks like fs_dax_get_by_bdev() is an intercept point where a > dax_device could be dynamically created to represent the subset range > indicated by the block-device partition. That would open up more > cleanup opportunities. Hi Dan, After a long time I got time to look at it again. Want to work on this cleanup so that I can make progress with virtiofs DAX paches. I am not sure I understand the requirements fully. I see that right now dax_device is created per device and all block partitions refer to it. If we want to create one dax_device per partition, then it looks like this will be structured more along the lines how block layer handles disk and partitions. (One gendisk for disk and block_devices for partitions, including partition 0). That probably means state belong to whole device will be in common structure say dax_device_common, and per partition state will be in dax_device and dax_device can carry a pointer to dax_device_common. I am also not sure what does it mean to partition dax devices. How will partitions be exported to user space. Thanks Vivek
