Re: [PATCH v2 08/17] fs: retrofit old error reporting API onto new infrastructure

[Jeff Layton <jlayton@redhat.com>]

On Mon, 2017-04-24 at 08:38 +1000, NeilBrown wrote:
> On Fri, Apr 21 2017, Jeff Layton wrote:
> 
> > On Tue, 2017-04-18 at 08:56 +1000, NeilBrown wrote:
> > > On Wed, Apr 12 2017, Jeff Layton wrote:
> > > 
> > > > On Thu, 2017-04-13 at 08:14 +1000, NeilBrown wrote:
> > > > > 
> > > > > I suspect that the filemap_check_wb_error() will need to be moved
> > > > > into some parent of the current call site, which is essentially what you
> > > > > suggest below.  It would be nice if we could do that first, rather than
> > > > > having the current rather odd code.  But maybe this way is an easier
> > > > > transition.  It isn't obviously wrong, it just isn't obviously right
> > > > > either.
> > > > > 
> > > > 
> > > > Yeah. It's just such a daunting task to have to change so much of the
> > > > existing code. I'm looking for ways to make this simpler.
> > > > 
> > > > I think it probably is reasonable for filemap_write_and_wait* to just
> > > > sample it as early as possible in those functions. filemap_fdatawait is
> > > > the real questionable one, as you may have already had some writebacks
> > > > complete with errors.
> > > > 
> > > > In any case, my thinking was that the old code is not obviously correct
> > > > either, so while this shortens the "error capture window" on these
> > > > calls, it seems like a reasonable place to start improving things.
> > > 
> > > I agree.  It wouldn't hurt to add a note to this effect in the patch
> > > comment so that people understand that the code isn't seen to be
> > > "correct" but only "no worse" with clear direction on what sort of
> > > improvement might be appropriate.
> > > 
> > 
> > I've got a cleaned-up set that is getting close to ready for
> > reposting. Before I do though, I think there is another option here
> > that's worth discussing.
> > 
> > We could store a second wb_err_t (aka errseq_t in the new set) in the
> > mapping that would would basically act as a "cursor" for these cases.
> > filemap_check_errors would need to do something like 
> > filemap_report_wb_error, but it would swap the value into the mapping's
> > cursor instead of dealing with the one in struct file.
> > 
> > I don't really like adding yet another field here, but the struct
> > address_space definition has this:
> > 
> >     __attribute__((aligned(sizeof(long))));
> > 
> > Adding the wb_err field means that we end up growing the struct by 8
> > bytes on x86_64 anyway. Adding another 4 bytes would just consume the
> > pad, so it wouldn't cost anything there. YMMV on other arches of
> > course.
> > 
> > That's also not perfectly like what we have with AS_EIO/AS_ENOSPC
> > flags, but is probably close enough not to matter.
> > 
> > So...this would let us limp along for even longer with the model of
> > reporting since last check. I'm not sure that's a good thing though. A
> > long term goal here is to have kernel code that's dealing with
> > writeback be more deliberate about the point from which it's checking
> > errors, and this doesn't help promote that.
> 
> I think this question needs some input from filesystem developers who
> might be affected by the answer.
> 
> My preference is to not add this field.  I think we would eventually
> want to remove it again, and it is easier to ensure it doesn't stay
> forever if it is never added.
> The version without this field isn't (I think) too bad, but maybe it is
> bad enough to motivate fs developers to create a better solution in each
> individual case.
> 
> If some filesystem developer says they don't like that sort of social
> engineering, or objects for any other reason, I will bow to the superior
> stake they hold.
> 
> 

That's pretty much my view too. I just figured I needed to throw the
option out there in the interest of full disclosure.

I think keeping a per-mapping cursor like this does make sense in some
situations though. For instance, there does seem to be quite a bit of
local fs journaling code that goes through the pagecache. For those, I
could see keeping the cursor in some sort of per-journal structure, and
doing a check-and-advance against that in appropriate places.

This is an option we can bring up for folks who do want to continue to
use a similar error tracking model in these situations though.
-- 
Jeff Layton <jlayton@redhat.com>