[RFD] What error should FS return when I/O failure occurs?

[fs <fs@ercist.iscas.ac.cn>]

Hi, ML subscribers,
    
    I'm taking part in the project DOUBT[1], and my sub-project
focuses on the consistency and coherency of FS[2].
    There is something I'm still confused - What error should FS
return when I/O failure occurs? It seems there are no relevant 
documents or standards on this issue.
    
    I'll just show some examples to make things clear:
1. For EXT3 partition , we mount it as RW, but when I/O occurs, the
   I/O related functions return EROFS(ReadOnly?), while other FSes
   return EIO.
2. Assume a program doing the following: open - write(async) - close
   When user-mode app calls sys_write, for EXT2/JFS, no error 
   returns, for EXT3, EROFS returns, for XFS/ReiserFS, EIO returns.

I know each FS has its own implementation, but from users'
perspective, they don't care what FS they're using. So, when 
handling errors from syscall, they can't do the following(p-code):
  ret = sys_write(fd, buf, size);
  if(ret < 0){
     /* the following is I/O failure related. */
     if((IsEXT3() && errno == EROFS) || 
        ((IsXFS() || IsReiserFS()) && errno == EIO)){
        /* do some things about I/O failure */
        ....
     }else{
        ....
     }
   } 
When I/O failure occurs, there should be some standards which 
define the ONLY error that should be returned from VFS, right?
What they should do is (p-code):
  ret = sys_write(fd, buf, size);
  if(ret < 0){
     if(errno == EIO){
        ...
     }else
        ...
  } 
   

Ref
[1]http://developer.osdl.jp/projects/doubt/
[2]http://developer.osdl.jp/projects/doubt/fs-consistency-and-coherency/index.html

regards,
----
Qu Fuping


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