Re: [PATCH] fsync.2: ERRORS: add EIO and ENOSPC

From: Michael Kerrisk (man-pages)
Date: Wed Sep 09 2020 - 11:08:33 EST

[CC += Neil, since he wrote the text we're talking about]

Hello Jan,

On 9/9/20 1:21 PM, Jan Kara wrote:
> On Wed 09-09-20 12:52:48, Michael Kerrisk (man-pages) wrote:
>>> So the error state isn't really stored "on pages in the file mapping".
>>> Current implementation (since 4.14) is that error state is stored in struct
>>> file (I think this tends to be called "file description" in manpages) and
>> (Yes, "open file description" is the POSIX terminology for the thing that
>> sits between the FD and the inode--struct file in kernel parlance--and I
>> try to follow POSIX terminology in the manual pages where possible.
>>> so EIO / ENOSPC is reported once for each file description of the file that
>>> was open before the error happened. Not sure if we want to be so precise in
>>> the manpages or if it just confuses people.
>> Well, people are confused now, so I think more detail would be good.
>>> Anyway your takeway that no
>>> error on subsequent fsync() does not mean data was written is correct.
>> Thanks. (See also my rply to Jeff.)
>> By the way, a question related to your comments above. In the
>> errors section, there is this:
>> EIO An error occurred during synchronization. This error may
>> relate to data written to some other 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. 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 that were open on the
>> * file when the error was recorded.
>> In the marked (*) lines, we have the word "file". Is this accurate? I mean, I
>> would normally take "file" in this context to mean the inode ('struct inode').
>> But I wonder if really what is meant here is "open file description"
>> ('struct file'). In other words, is the EIO being generated for all FDs
>> connected to the same open file description, or for all FDs for all of the
>> open file descriptions connected to the inode? Your thoughts?
> The error gets reported once for each "open file description" of the file
> (inode) where the error happened. If there are multiple file descriptors
> pointing to the same open file description, then only one of those file
> descriptors will see the error. This is inevitable consequence of kernel
> storing the error state in struct file and clearing it once it is
> reported...

So, the text in wrong two respects, I believe:

* It should be phrased in terms of "open file description", not "file",
in the lines that I marked.

* Where it says "to all file descriptors" (twice), it should rather say
"to any of the file descriptors [that refer to the open file description]"

Do you agree?



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training: