I liked the patch you were pushing to request the *page* containing the requested bytes instead of the *block* containing the requested bytes. For the misaligned partition problem, I was thinking we should change the direct_access API to return a phys_addr_t instead of a pfn. That way we can return something that isn't actually page aligned, and DAX can take care of making sure it doesn't overshoot the end. -----Original Message----- From: Jeff Moyer [mailto:jmoyer@xxxxxxxxxx] Sent: Thursday, August 13, 2015 11:19 AM To: Linda Knippers Cc: Boaz Harrosh; Wilcox, Matthew R; linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx; Verma, Vishal L Subject: Re: regression introduced by "block: Add support for DAX reads/writes to block devices" Linda Knippers <linda.knippers@xxxxxx> writes: >>> It causes the physical block size to be PAGE_SIZE but the >>> logical block size is still 512. However, the minimum_io_size >>> is now 4096 (same as physical block size, I assume). The >>> optimal_io_size is still 0. What does that mean? >> >> physical block size - device's internal block size >> logical block size - addressable unit > > Right, but it's still reported as 512 and that doesn't work. Understood. :) >> optimal io size - device's preferred unit for streaming > > So 0 is ok. Correct. >> We can change the block device to export logical/physical block sizes of >> PAGE_SIZE. However, when persistent memory support comes to platforms >> that support page sizes > 32k, xfs will again run into problems (Dave >> Chinner mentioned that xfs can't deal with logical block sizes >32k.) >> Arguably, you can use pmem and dax on such platforms using RAM today for >> testing. Do we care about breaking that? > > I would think so. AARCH64 uses 64k pages today. So does powerpc, but I guess nobody cares about that anymore. ;-) If the logical block size is smaller than the page size, we're going to have to deal with sub-page I/O. For now, we can do as Boaz suggested, and just turn off dax for those configurations. We could also just revert the patch that introduced this problem. I really don't know who is going to care about O_DIRECT I/O performance to a persistent memory block device. Willy? What was the real motivation there? > I think Documentation/filesystems/dax.txt could use a little update > too. It has a section "Implementation Tips for Block Driver Writers" > that makes it sound easy but now I wonder if it even works with the > example ram drivers. Should we be able to read any 512 byte > "sector"? If the logical block size is 512 bytes, then you have to be able to do (direct) I/O to any 512 byte sector. Simple as that. Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
