Re: [Lsf-pc] [LSF/MM TOPIC] do we really need PG_error at all?

[Jeff Layton]

On Tue, 2017-02-28 at 12:12 +0200, Boaz Harrosh wrote:
> On 02/28/2017 03:11 AM, Jeff Layton wrote:
> <>
> > 
> > I'll probably have questions about the read side as well, but for now it
> > looks like it's mostly used in an ad-hoc way to communicate errors
> > across subsystems (block to fs layer, for instance).
> 
> If memory does not fail me it used to be checked long time ago in the
> read-ahead case. On the buffered read case, the first page is read synchronous
> and any error is returned to the caller, but then a read-ahead chunk is
> read async all the while the original thread returned to the application.
> So any errors are only recorded on the page-bit, since otherwise the uptodate
> is off and the IO will be retransmitted. Then the move to read_iter changed
> all that I think.
> But again this is like 5-6 years ago, and maybe I didn't even understand
> very well.
> 

Yep, that's what I meant about using it to communicate errors between
layers. e.g. end_buffer_async_read will check PageError and only
SetPageUptodate if it's not set. That has morphed a lot in the last few
years though and it looks like it may rely on PG_error less than it used
to.

> 
> I would like a Documentation of all this as well please. Where are the
> tests for this?
> 

Documentation is certainly doable (and I'd like to write some once we
have this all straightened out). In particular, I think we need clear
guidelines for fs authors on how to handle pagecache read and write
errors. Tests are a little tougher -- this is all kernel-internal stuff
and not easily visible to userland.

The one thing I have noticed is that even if you set AS_ENOSPC in the
mapping, you'll still get back -EIO on the first fsync if any PG_error
bits are set. I think we ought to fix that by not doing the
TestClearPageError call in __filemap_fdatawait_range, and just rely on
the mapping error there.

We could maybe roll a test for that, but it's rather hard to test ENOSPC
conditions in a fs-agnostic way. I'm open to suggestions here though.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>