linux-kernel - Re: [PATCH -next,v2 0/3] nfs: handle writeback errors correctly

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Date:   Fri, 1 Apr 2022 15:03:34 +0800
From:   "chenxiaosong (A)" <chenxiaosong2@...wei.com>
To:     <trond.myklebust@...merspace.com>, <anna@...nel.org>,
        <smayhew@...hat.com>
CC:     <linux-nfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <liuyongqiang13@...wei.com>, <yi.zhang@...wei.com>,
        <zhangxiaoxu5@...wei.com>
Subject: Re: [PATCH -next,v2 0/3] nfs: handle writeback errors correctly

在 2022/4/1 11:44, ChenXiaoSong 写道:
> v1:
> cover letter: (nfs: check writeback errors correctly)
> 
> v2:
> - return more nuanced writeback errors in nfs_file_write().
> - return writeback error in close()->flush() without consumed it.
> - fix: nfs_file_write() will always call nfs_wb_all() even if there is no
> new writeback error.
> 
> 
> ChenXiaoSong (3):
>    NFS: return more nuanced writeback errors in nfs_file_write()
>    NFS: nfs{,4}_file_flush() return correct writeback errors
>    Revert "nfs: nfs_file_write() should check for writeback errors"
> 
>   fs/nfs/file.c     | 23 ++++++++++-------------
>   fs/nfs/nfs4file.c |  8 ++++----
>   fs/nfs/write.c    |  5 +----
>   3 files changed, 15 insertions(+), 21 deletions(-)
> 

It is not a good idea to modify error sequence mechanism, as the 
`lib/errseq.c` described:

	22  * Note that there is a risk of collisions if new errors are being 
recorded
	23  * frequently, since we have so few bits to use as a counter. 

	24  *
	25  * To mitigate this, one bit is used as a flag to tell whether the 
value has
	26  * been sampled since a new value was recorded. That allows us to 
avoid bumping
	27  * the counter if no one has sampled it since the last time an error was
	28  * recorded.


So, if we want to report nuanced writeback error, it is better to detect 
wb error from filemap_check_errors(), and then return 
-(file->f_mapping->wb_err & MAX_ERRNO) to userspace without consume it.

   nfs_mapping_set_error
     mapping_set_error
       __filemap_set_wb_err // record error sequence
         errseq_set
       set_bit(..., &mapping->flags) // record address_space flag

   // it is not used to be reported, just used to detect
   error = filemap_check_errors // -ENOSPC or -EIO
     test_and_clear_bit(..., &mapping->flags) // error bit cleared

   // now we try to return nuanced writeback error
   if (error)
   return filemap_check_wb_err(file->f_mapping, 0);
     return -(file->f_mapping->wb_err & MAX_ERRNO)