Re: [PATCH 2/5] kernel.h: Add non_block_start/end()

Michal Hocko
2019-08-20T08:18:29Z

On Fri 16-08-19 11:31:45, Jason Gunthorpe wrote:
> On Fri, Aug 16, 2019 at 02:26:25PM +0200, Michal Hocko wrote:
[...]
> > I believe I have given some examples when introducing __GFP_NOLOCKDEP.
>
> Okay, I think that is 7e7844226f10 ("lockdep: allow to disable reclaim
> lockup detection") Hmm, sadly the lkml link in the commit is broken.
>
> Hum. There are no users of __GFP_NOLOCKDEP today?? Could all the false
> positives have been fixed??

I would be more than surprised. Those false possitives were usually
papered over by using GFP_NOFS. And the original plan was to convert
those back to GFP_KERNEL like allocations and use __GFP_NOLOCKDEP.

> Keep in mind that this fs_reclaim was reworked away from the hacky
> interrupt context flag to a fairly elegant real lock map.

I am glad to hear that because that was just too ugly to live.

> Based on my read of the commit message, my first reaction would be to
> suggest lockdep_set_class() to solve the problem described, ie
> different classes for 'inside transaction' and 'outside transaction'
> on xfs_refcountbt_init_cursor()

No this just turned out to be unmaintainable. The number of lock classes
was growing high. I recommend on of the Dave Chinner's rant. Sorry not
link handy.

> I understood that generally that is the way to handle lockdep false
> positives.
>
> Anyhow, if you are willing to consider that lockdep isn't broken, I
> have some ideas on how to make this clearer and increase
> coverage. Would you be willing to look at patches on this topic? (not
> soon, I have to finish my mmu notifier stuff)

I haven't claimed that the lockdep is broken. It just had problems to
capture some code paths and generated false positives. I would recommend
talking to lockdep maintainers much more than to me because I would have
to dive into the code much more to be useful. I can still comment on the
MM side of the thing of course if that is helpful.

> > > I would like to inject it into the notifier path as this is very
> > > difficult for driver authors to discover and know about, but I'm
> > > worried about your false positive remark.
> > >
> > > I think I understand we can use only GFP_ATOMIC in the notifiers, but
> > > we need a strategy to handle OOM to guarentee forward progress.
> >
> > Your example is from the notifier registration IIUC.
>
> Yes, that is where this commit hit it.. Triggering this under an
> actual notifier to get a lockdep report is hard.

All you need is to generate a memory pressure. That shouldn't be that
hard.
--
Michal Hocko
SUSE Labs