Re: [RFC PATCH manpages] write.2, fsync.2, close.2: update description of error codes

Michael Kerrisk (man-pages)
2017-09-15T07:50:28Z

Hi Neil,

Will you revise this patch to incorporate Jeff's comments, or
should I try manually editing them in. (I'd prefer the former.)

Cheers,

Michael

On 09/14/2017 12:48 PM, Jeff Layton wrote:
> On Thu, 2017-09-14 at 09:50 +1000, NeilBrown wrote:
>> Since 4.13, errors from writeback are more reliably reported
>> to all file descriptors that might be relevant.
>>
>> Add notes to this effect, and also add details about ENOSPC and EDQUOT
>> which can be delayed in a similar manner to EIO - for NFS in particular.
>>
>> Signed-off-by: NeilBrown <neilb@suse.com>
>> ---
>>
>> This is my summary of recent changes, and details that have been made
>> clear during the exploration of those changes.
>>
>> I haven't mentioned the fact that EPERM can be returned by
>> write/fsync/close on NFS if the permissions on the server are changed.
>> We probably should ... are there other errors that are worth mentioning
>> along with EPERM, ENOSPC, EDQUOT ??
>>
>> Thanks,
>> NeilBronw
>>
>
> Many thanks for doing this! It was on my to-do list. Comments below:
>
>>
>> man2/close.2 | 9 +++++++++
>> man2/fsync.2 | 19 ++++++++++++++++++-
>> man2/write.2 | 20 +++++++++++++++++---
>> 3 files changed, 44 insertions(+), 4 deletions(-)
>>
>> diff --git a/man2/close.2 b/man2/close.2
>> index 751ec322b1f1..9776c839b8b6 100644
>> --- a/man2/close.2
>> +++ b/man2/close.2
>> @@ -82,6 +82,15 @@ call was interrupted by a signal; see
>> .TP
>> .B EIO
>> An I/O error occurred.
>> +.TP
>> +.BR ENOSPC ", " EDQUOT
>> +On NFS, these errors are not normally reported against the first write
>> +which exceeds the available storage space, but instead against a
>> +subsequent
>> +.BR write (2),
>> +.BR fsync (2),
>> +or
>> +.BR close (2).
>> .PP
>> See NOTES for a discussion of why
>> .BR close ()
>> diff --git a/man2/fsync.2 b/man2/fsync.2
>> index f1a01301da0f..e706a08d360d 100644
>> --- a/man2/fsync.2
>> +++ b/man2/fsync.2
>> @@ -120,12 +120,29 @@ is set appropriately.
>> is not a valid open file descriptor.
>> .TP
>> .B EIO
>> -An error occurred during synchronization.
>> +An error occurred during synchronization. This error may relate
>> +to data written to some other file descriptor on the same file.
>> +.\" commit 088737f44bbf6378745f5b57b035e57ee3dc4750
>> +Since Linux 4.13 errors from write-back will be reported to
>> +all file descriptors that might have written the data which triggered
>> +the error, and which are still open.
>
> This is a little awkward. How could we report to a fd that was no longer
> open? How about:
>
> "Since Linux 4.13, errors from write-back will be reported to all file
> descriptors that were open at the time that the error was recorded."
>
>> Some filesystems (e.g. NFS)
>> +keep close track of which data came through which file descriptor,
>> +and give more precise reporting. Other filesystems (e.g. most local
>> +filesystems) will report errors to all file descriptors on the same
>> +file.
>> .TP
>> .BR EROFS ", " EINVAL
>> .I fd
>> is bound to a special file (e.g., a pipe, FIFO, or socket)
>> which does not support synchronization.
>> +.TP
>> +.BR ENOSPC ", " EDQUOT
>> +.I fd
>> +is bound to a file on NFS or another filesystem which does not allocate
>> +space at the time of a
>> +.BR write (2)
>> +system call, and some previous write failed due to insufficient
>> +storage space.
>> .SH CONFORMING TO
>> POSIX.1-2001, POSIX.1-2008, 4.3BSD.
>> .SH AVAILABILITY
>> diff --git a/man2/write.2 b/man2/write.2
>> index 6a39b5b5541d..1a9a86b03b04 100644
>> --- a/man2/write.2
>> +++ b/man2/write.2
>> @@ -47,7 +47,7 @@ write \- write to a file descriptor
>> .BR write ()
>> writes up to
>> .I count
>> -bytes from the buffer pointed
>> +bytes from the buffer starting at
>> .I buf
>> to the file referred to by the file descriptor
>> .IR fd .
>> @@ -181,6 +181,14 @@ or the file offset is not suitably aligned.
>> .TP
>> .B EIO
>> A low-level I/O error occurred while modifying the inode.
>> +This error may relate to data written by an earlier
>> +.BR write (2),
>> +which may have been issued to a different file descriptor on
>> +the same file. Since Linux 4.13 errors from write-back will
>> +be reported to all file descriptors that might have
>> +written the data which triggered the error, and which are still
>> +open.
>
>
> This is where things get a little more vague.
>
> Some filesystems will return errors on a subsequent write(2) when
> previous writeback has failed -- some don't. In either case though,
> write(2) should never advance your errseq_t cursor, so only an fsync
> will "clear" an earlier error.
>
> I'm not sure how best to convey that in the manpages though.
>
>> +.\" commit 088737f44bbf6378745f5b57b035e57ee3dc4750
>> .TP
>> .B ENOSPC
>> The device containing the file referred to by
>> @@ -222,8 +230,14 @@ unsigned and signed integer data types specified by POSIX.1.
>> A successful return from
>> .BR write ()
>> does not make any guarantee that data has been committed to disk.
>> -In fact, on some buggy implementations, it does not even guarantee
>> -that space has successfully been reserved for the data.
>> +On some filesystems, including NFS, it does not even guarantee
>> +that space has successfully been reserved for the data. In the case,
>> +some errors might be delayed to a future
>> +.BR write (2)
>> +or to
>> +.BR fsync (2)
>> +or even
>> +.BR close (2).
>> The only way to be sure is to call
>> .BR fsync (2)
>> after you are done writing all your data.
>

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/