On Mon, May 2, 2016 at 10:44 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote: > 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. The semantic I am talking about preserving is: buffered / unaligned write of a bad sector => -EIO on reading into the page cache ...and that the only guaranteed way to clear an error (assuming the block device supports it) is an O_DIRECT write. > > 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. Where do you get the rest of the data to complete a full page write? > So reads return -EIO - is what you wanted no? That's well understood. What we are debating is the method to clear errors / ask the storage device to remap bad blocks. > 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) See all the previous discussions on why the fallback path is problematic to implement. > > [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 I still think we're talking past each other on this point. This patch set is not overloading error semantics, it's fixing the error handling problem that was introduced in this commit: d475c6346a38 dax,ext2: replace XIP read and write with DAX I/O ...where we started overloading O_DIRECT and dax_do_io() semantics. -- 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
