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

[Dan Williams]

On Wed, May 23, 2018 at 9:47 AM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> 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
>> <ross.zwisler@xxxxxxxxxxxxxxx> 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: <x86@xxxxxxxxxx>
>> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> >> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> >> Cc: Tony Luck <tony.luck@xxxxxxxxx>
>> >> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>> >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> >> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> >> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> >> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> >> ---
>> >>  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.