On 05/02/2016 07:49 PM, Dan Williams wrote: > On Mon, May 2, 2016 at 9:22 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote: >> On 05/02/2016 07:01 PM, Dan Williams wrote: >>> On Mon, May 2, 2016 at 8:41 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote: >>>> On 04/29/2016 12:16 AM, Vishal Verma wrote: >>>>> All IO in a dax filesystem used to go through dax_do_io, which cannot >>>>> handle media errors, and thus cannot provide a recovery path that can >>>>> send a write through the driver to clear errors. >>>>> >>>>> Add a new iocb flag for DAX, and set it only for DAX mounts. In the IO >>>>> path for DAX filesystems, use the same direct_IO path for both DAX and >>>>> direct_io iocbs, but use the flags to identify when we are in O_DIRECT >>>>> mode vs non O_DIRECT with DAX, and for O_DIRECT, use the conventional >>>>> direct_IO path instead of DAX. >>>>> >>>> >>>> Really? What are your thinking here? >>>> >>>> What about all the current users of O_DIRECT, you have just made them >>>> 4 times slower and "less concurrent*" then "buffred io" users. Since >>>> direct_IO path will queue an IO request and all. >>>> (And if it is not so slow then why do we need dax_do_io at all? [Rhetorical]) >>>> >>>> I hate it that you overload the semantics of a known and expected >>>> O_DIRECT flag, for special pmem quirks. This is an incompatible >>>> and unrelated overload of the semantics of O_DIRECT. >>> >>> I think it is the opposite situation, it us undoing the premature >>> overloading of O_DIRECT that went in without performance numbers. >> >> We have tons of measurements. Is not hard to imagine the results though. >> Specially the 1000 threads case >> >>> This implementation clarifies that dax_do_io() handles the lack of a >>> page cache for buffered I/O and O_DIRECT behaves as it nominally would >>> by sending an I/O to the driver. >> >>> It has the benefit of matching the >>> error semantics of a typical block device where a buffered write could >>> hit an error filling the page cache, but an O_DIRECT write potentially >>> triggers the drive to remap the block. >>> >> >> I fail to see how in writes the device error semantics regarding remapping of >> blocks is any different between buffered and direct IO. As far as the block >> device it is the same exact code path. All The big difference is higher in the >> VFS. >> >> And ... So you are willing to sacrifice the 99% hotpath for the sake of the >> 1% error path? and piggybacking on poor O_DIRECT. >> >> Again there are tons of O_DIRECT apps out there, why are you forcing them to >> change if they want true pmem performance? > > This isn't forcing them to change. This is the path of least surprise > as error semantics are identical to a typical block device. Yes, an > application can go faster by switching to the "buffered" / dax_do_io() > path it can go even faster to switch to mmap() I/O and use DAX > directly. If we can later optimize the O_DIRECT path to bring it's > performance more in line with dax_do_io(), great, but the > implementation should be correct first and optimized later. > Why does it need to be either or. Why not both? And also I disagree if you are correct and dax_do_io is bad and needs fixing than you have broken applications. Because in current model: read => -EIO, write-bufferd, sync() gives you the same error semantics as: read => -EIO, write-direct-io In fact this is what the delete, restore from backup model does today. Who said it uses / must direct IO. Actually I think it does not. Two things I can think of which are better: [1] Why not go deeper into the dax io loops, and for any WRITE failed page call bdev_rw_page() to let the pmem.c clear / relocate the error page. So reads return -EIO - is what you wanted no? writes get a memory error and retry with bdev_rw_page() to let the bdev relocate / clear the error - is what you wanted no? In the partial page WRITE case on bad sectors. we can carefully read-modify-write sector-by-sector and zero-out the bad-sectors that could not be read, what else? (Or enhance the bdev_rw_page() API) [2] Only switch to slow O_DIRECT, on presence of errors like you wanted. But I still hate that you overload error semantics with O_DIRECT which does not exist today see above Thanks Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
