On Thu, May 5, 2016 at 8:22 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Thu, May 05, 2016 at 08:15:32AM -0700, Dan Williams wrote: >> > Agreed - makig O_DIRECT less direct than not having it is plain stupid, >> > and I somehow missed this initially. >> >> Of course I disagree because like Dave argues in the msync case we >> should do the correct thing first and make it fast later, but also >> like Dave this arguing in circles is getting tiresome. > > We should do the right thing first, and make it fast later. But this > proposal is not getting it right - it still does not handle errors > for the fast path, but magically makes it work for direct I/O by > in general using a less optional path for O_DIRECT. It's getting the > worst of all choices. > > As far as I can tell the only sensible option is to: > > - always try dax-like I/O first > - have a custom get_user_pages + rw_bytes fallback handles bad blocks > when hitting EIO If you're on board with more special fallbacks for dax-capable block devices that indeed opens up the thinking. The O_DIRECT approach was meant to keep the error clearing model close to the traditional block device case, but yes that does constrain the implementation in sub-optimal ways. However, we still have the alignment problem in the rw_bytes case, how do we communicate to the application that only writes with a certain size/alignment will clear errors? That forced alignment assumption was the other appeal of O_DIRECT. Perhaps we can at least start with hole punching and block reallocation as the error clearing method while we think more about the write-to-clear case? > And then we need to sort out the concurrent write synchronization. > Again there I think we absolutely have to obey Posix for the !O_DIRECT > case and can avoid it for O_DIRECT, similar to the existing non-DAX > semantics. If we want any special additional semantics we _will_ need > a special O_DAX flag. Ok, makes sense. -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
