linux-kernel - Re: [PATCH 0/3] mm/fs: get PG_error out of the writeback reporting business

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Date:   Mon, 06 Mar 2017 14:06:11 +1100
From:   NeilBrown <neilb@...e.com>
To:     Jeff Layton <jlayton@...hat.com>, viro@...iv.linux.org.uk,
        konishi.ryusuke@....ntt.co.jp
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-nilfs@...r.kernel.org
Subject: Re: [PATCH 0/3] mm/fs: get PG_error out of the writeback reporting business

On Sun, Mar 05 2017, Jeff Layton wrote:

> I recently did some work to wire up -ENOSPC handling in ceph, and found
> I could get back -EIO errors in some cases when I should have instead
> gotten -ENOSPC. The problem was that the ceph writeback code would set
> PG_error on a writeback error, and that error would clobber the mapping
> error.
>
> While I fixed that problem by simply not setting that bit on errors,
> that led me down a rabbit hole of looking at how PG_error is being
> handled in the kernel.

Speaking of rabbit holes... I thought to wonder how IO error propagate
up from NFS.
It doesn't use SetPageError or mapping_set_error() for files (except in
one case that looks a bit odd).
It has an "nfs_open_context" and store the latest error in ctx->error.

So when you get around to documenting how this is supposed to work, it
would be worth while describing the required observable behaviour, and
note that while filesystems can use mapping_set_error() to achieve this,
they don't have to.

I notice that
  drivers/staging/lustre/lustre/llite/rw.c
  fs/afs/write.c
  fs/btrfs/extent_io.c
  fs/cifs/file.c
  fs/jffs2/file.c
  fs/jfs/jfs_metapage.c
  fs/ntfs/aops.c

(and possible others) all have SetPageError() calls that seem to be
in response to a write error to a file, but don't appear to have
matching mapping_set_error() calls.  Did you look at these?  Did I miss
something?

Thanks,
NeilBrown

>
> This patch series is a few fixes for things that I 100% noticed by
> inspection. I don't have a great way to test these since they involve
> error handling. I can certainly doctor up a kernel to inject errors
> in this code and test by hand however if these look plausible up front.
>
> Jeff Layton (3):
>   nilfs2: set the mapping error when calling SetPageError on writeback
>   mm: don't TestClearPageError in __filemap_fdatawait_range
>   mm: set mapping error when launder_pages fails
>
>  fs/nilfs2/segment.c |  1 +
>  mm/filemap.c        | 19 ++++---------------
>  mm/truncate.c       |  6 +++++-
>  3 files changed, 10 insertions(+), 16 deletions(-)
>
> -- 
> 2.9.3

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