On Wed, May 23, 2018 at 09:39:04AM -0700, Dan Williams wrote: > On Wed, May 23, 2018 at 9:34 AM, Ross Zwisler > <ross.zwis...@linux.intel.com> wrote: > > On Thu, May 03, 2018 at 05:06:42PM -0700, Dan Williams wrote: > >> In preparation for protecting the dax read(2) path from media errors > >> with copy_to_iter_mcsafe() (via dax_copy_to_iter()), convert the > >> implementation to report the bytes successfully transferred. > >> > >> Cc: <x...@kernel.org> > >> Cc: Ingo Molnar <mi...@redhat.com> > >> Cc: Borislav Petkov <b...@alien8.de> > >> Cc: Tony Luck <tony.l...@intel.com> > >> Cc: Al Viro <v...@zeniv.linux.org.uk> > >> Cc: Thomas Gleixner <t...@linutronix.de> > >> Cc: Andy Lutomirski <l...@amacapital.net> > >> Cc: Peter Zijlstra <pet...@infradead.org> > >> Cc: Andrew Morton <a...@linux-foundation.org> > >> Cc: Linus Torvalds <torva...@linux-foundation.org> > >> Signed-off-by: Dan Williams <dan.j.willi...@intel.com> > >> --- > >> fs/dax.c | 20 +++++++++++--------- > >> 1 file changed, 11 insertions(+), 9 deletions(-) > >> > >> diff --git a/fs/dax.c b/fs/dax.c > >> index a64afdf7ec0d..34a2d435ae4b 100644 > >> --- a/fs/dax.c > >> +++ b/fs/dax.c > >> @@ -991,6 +991,7 @@ dax_iomap_actor(struct inode *inode, loff_t pos, > >> loff_t length, void *data, > >> struct iov_iter *iter = data; > >> loff_t end = pos + length, done = 0; > >> ssize_t ret = 0; > >> + size_t xfer; > >> int id; > >> > >> if (iov_iter_rw(iter) == READ) { > >> @@ -1054,19 +1055,20 @@ dax_iomap_actor(struct inode *inode, loff_t pos, > >> loff_t length, void *data, > >> * vfs_write(), depending on which operation we are doing. > >> */ > >> if (iov_iter_rw(iter) == WRITE) > >> - map_len = dax_copy_from_iter(dax_dev, pgoff, kaddr, > >> + xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr, > >> map_len, iter); > >> else > >> - map_len = dax_copy_to_iter(dax_dev, pgoff, kaddr, > >> + xfer = dax_copy_to_iter(dax_dev, pgoff, kaddr, > >> map_len, iter); > >> - if (map_len <= 0) { > >> - ret = map_len ? map_len : -EFAULT; > >> - break; > >> - } > >> > >> - pos += map_len; > >> - length -= map_len; > >> - done += map_len; > >> + pos += xfer; > >> + length -= xfer; > >> + done += xfer; > >> + > >> + if (xfer == 0) > >> + ret = -EFAULT; > >> + if (xfer < map_len) > >> + break; > > > > I'm confused by this error handling. So if we hit an error on a given iov > > and > > we don't transfer the expected number of bytes, we have two cases: > > > > 1) We transferred *something* on this iov but not everything - return > > success. > > 2) We didn't transfer anything on this iov - return -EFAULT. > > > > Both of these are true regardless of data transferred on previous iovs. > > > > Why the distinction? If a given iov is interrupted, regardless of whether > > it > > transferred 0 bytes or 1, shouldn't the error path be the same? > > This is is the semantics of read(2) / write(2). Quoting the pwrite man page: > > Note that is not an error for a successful call to > transfer fewer bytes than > requested (see read(2) and write(2)).
Consider this case: I have 4 IOVs, each of a full page. The first three transfer their full page, but on the third we hit an error. If we transferred 0 bytes in the fourth page, we'll return -EFAULT. If we transferred 1 byte in the fourth page, we'll return the total length transferred, so 3 pages + 1 byte. Why? pwrite(2) says it returns the number of bytes written, which can be less than the total requested. Why not just return the length transferred in both cases, instead of returning -EFAULT for one of them?