On Mon, Dec 15, 2014 at 06:16:02AM +0000, Al Viro wrote: > On Sun, Dec 14, 2014 at 09:26:57PM -0800, Omar Sandoval wrote: > > The rw argument to direct_IO has some ill-defined semantics. Some > > filesystems (e.g., ext4, FAT) decide whether they're doing a write with > > rw == WRITE, but others (e.g., XFS) check rw & WRITE. Let's set a good > > example in the swap file code and say ITER_BVEC belongs in > > iov_iter->flags but not in rw. This caters to the least common > > denominator and avoids a sweeping change of every direct_IO > > implementation for now. > > Frankly, this is bogus. If anything, let's just kill the first argument > completely - ->direct_IO() can always pick it from iter->type. > > As for catering to the least common denominator... To hell with the lowest > common denominator. How many instances of ->direct_IO() do we have, anyway? > 24 in the mainline (and we don't give a flying fuck for out-of-tree code, as > a matter of policy). Moreover, several are of "do nothing" variety. > > FWIW, 'rw' is a mess. We used to have this: > READ: O_DIRECT read > WRITE: O_DIRECT write > KERNEL_WRITE: swapout > > These days KERNEL_WRITE got replaced with ITER_BVEC | WRITE. The thing is, > we have a bunch of places where we explicitly checked for being _equal_ to > WRITE. I.e. the checks that gave a negative on swapouts. I suspect that most > of them are wrong and should trigger on all writes, including swapouts, but > I really didn't want to dig into that pile of fun back then. That's the > main reason why 'rw' argument has survived at all... > In that case, I'll take a stab at nuking rw. I'm almost certain that some of these are completely wrong (for example, of the form if (rw == WRITE) do_write(); else do_read();). This isn't an immediate problem for swap files on BTRFS, as __blockdev_direct_IO does a bitwise test, so I think I'll split it out into its own series. Thanks, -- Omar -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>