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

[Jeff Layton]

On Mon, 2017-03-06 at 14:06 +1100, NeilBrown wrote:
> 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?
> 

Those are all in writepage implementations, and the callers of writepage
all call mapping_set_error if it returns error. The exception is
write_one_page, which is typically used for writing out dir info and
such, and so it's not really necessary there.

Now that I look though, I think I may have gotten the page migration
codepath wrong. I had convinced myself it was ok before, but looking
again, I think we probably need to add a mapping_set_error call to 
writeout() as well. I'll go over that more carefully in a little while.

> > 
> > 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

-- 
Jeff Layton <jlayton@xxxxxxxxxx>