by Ross Zwisler

[permalink] [raw]

Subject: Re: [PATCH v3 7/9] dax: report bytes remaining in dax_iomap_actor()

On Wed, May 23, 2018 at 09:53:38AM -0700, Dan Williams wrote:
> On Wed, May 23, 2018 at 9:47 AM, Ross Zwisler
> <[email protected]> wrote:
> > On Wed, May 23, 2018 at 09:39:04AM -0700, Dan Williams wrote:
> >> On Wed, May 23, 2018 at 9:34 AM, Ross Zwisler
> >> <[email protected]> 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: <[email protected]>
> >> >> Cc: Ingo Molnar <[email protected]>
> >> >> Cc: Borislav Petkov <[email protected]>
> >> >> Cc: Tony Luck <[email protected]>
> >> >> Cc: Al Viro <[email protected]>
> >> >> Cc: Thomas Gleixner <[email protected]>
> >> >> Cc: Andy Lutomirski <[email protected]>
> >> >> Cc: Peter Zijlstra <[email protected]>
> >> >> Cc: Andrew Morton <[email protected]>
> >> >> Cc: Linus Torvalds <[email protected]>
> >> >> Signed-off-by: Dan Williams <[email protected]>
> >> >> ---
> >> >> 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?
>
> Ah, now I see. Yes, that's a bug. Once we have successfully completed
> any iovec we should be returning bytes transferred not -EFAULT.

Actually, your code is fine. This is handled by the:

return done ? done : ret;

at the end of the function. So if we've transferred any data at all, we'll
return the number of bytes transferred, and if we didn't we'll return -EFAULT
because 0 is the special case which means EOF according to pread(2)/pwrite(2).

Looks good, then. Thanks for answering my questions.

Reviewed-by: Ross Zwisler <[email protected]>