On Tue, Aug 25, 2015 at 10:00:58AM -0400, Jeff Moyer wrote: > Dave Chinner <david@xxxxxxxxxxxxx> writes: > > > On Mon, Aug 24, 2015 at 01:19:24PM -0400, Jeff Moyer wrote: > >> Brian Norris <computersforpeace@xxxxxxxxx> writes: > >> > >> > On Mon, Aug 24, 2015 at 10:13:25AM +0300, Artem Bityutskiy wrote: > >> >> Now, some user-space fails when direct I/O is not supported. > >> > > >> > I think the whole argument rested on what it means when "some user space > >> > fails"; apparently that "user space" is just a test suite (which > >> > can/should be fixed). > >> > >> Even if it wasn't a test suite it should still fail. Either the fs > >> supports O_DIRECT or it doesn't. Right now, the only way an application > >> can figure this out is to try an open and see if it fails. Don't break > >> that. > > > > Who cares how a filesystem implements O_DIRECT as long as it does > > not corrupt data? ext3 fell back to buffered IO in many situations, > > yet the only complaints about that were performance. IOWs, it's long been > > true that if the user cares about O_DIRECT *performance* then they > > have to be careful about their choice of filesystem. > > > But if it's only 5 lines of code per filesystem to support O_DIRECT > > *correctly* via buffered IO, then exactly why should userspace have > > to jump through hoops to explicitly handle open(O_DIRECT) failure? > > > Especially when you consider that all they can do is fall back to > > buffered IO themselves.... > > I had written counterpoints for all of this, but I thought better of > it. Old versions of the kernel simply ignore O_DIRECT, so clearly > there's precedent. > > I do think we should at least document what file systems appear to be > doing. Here's a man page patch for open (generated with extra context > for easier reading). Let me know what you think. We shouldn't be ignoring it, but instead call it similar to O_DSYNC plus removing the pages from cache. -chris -- 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
