Re: [PATCH 2/2] mm, debug: report when GFP_NO{FS,IO} is used explicitly from memalloc_no{fs,io}_{save,restore} context

[Dave Chinner]

On Tue, May 03, 2016 at 05:38:23PM +0200, Michal Hocko wrote:
> On Sat 30-04-16 09:40:08, Dave Chinner wrote:
> > On Fri, Apr 29, 2016 at 02:12:20PM +0200, Michal Hocko wrote:
> [...]
> > > - was it 
> > > "inconsistent {RECLAIM_FS-ON-[RW]} -> {IN-RECLAIM_FS-[WR]} usage"
> > > or a different class reports?
> > 
> > Typically that was involved, but it quite often there'd be a number
> > of locks and sometimes even interrupt stacks in an interaction
> > between 5 or 6 different processes. Lockdep covers all sorts of
> > stuff now (like fs freeze annotations as well as locks and memory
> > reclaim) so sometimes the only thing we can do is remove the
> > reclaim context from the stack and see if that makes it go away...
> 
> That is what I was thinking of. lockdep_reclaim_{disable,enable} or
> something like that to tell __lockdep_trace_alloc to not skip
> mark_held_locks(). This would effectivelly help to get rid of reclaim
> specific reports. It is hard to tell whether there would be others,
> though.

Yeah, though I suspect this would get messy having to scatter it
around the code. I can encapsulate it via internal XFS KM flags,
though, so I do think that will be a real issue.

> > > > They may have been fixed since, but I'm sceptical
> > > > of that because, generally speaking, developer testing only catches
> > > > the obvious lockdep issues. i.e. it's users that report all the
> > > > really twisty issues, and they are generally not reproducable except
> > > > under their production workloads...
> > > > 
> > > > IOWs, the absence of reports in your testing does not mean there
> > > > isn't a problem, and that is one of the biggest problems with
> > > > lockdep annotations - we have no way of ever knowing if they are
> > > > still necessary or not without exposing users to regressions and
> > > > potential deadlocks.....
> > > 
> > > I understand your points here but if we are sure that those lockdep
> > > reports are just false positives then we should rather provide an api to
> > > silence lockdep for those paths
> > 
> > I agree with this - please provide such infrastructure before we
> > need it...
> 
> Do you think a reclaim specific lockdep annotation would be sufficient?

It will help - it'll take some time to work through all the explicit
KM_NOFS calls in XFS, though, to determine if they are just working
around lockdep false positives or some other potential problem....

> I do understand your concerns and I really do not ask you to redesign
> your code. I would like make the code more maintainable and reducing the
> number of (undocumented) GFP_NOFS usage to the minimum seems to be like
> a first step. Now the direct usage of GFP_NOFS (resp. KM_NOFS) in xfs is
> not that large.

That's true, and if we can reduce them to real cases of GFP_NOFS
being needed vs annotations to silence lockdep false positives we'll
then know what problems we really need to fix...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx