Re: [PATCH 4.19 56/81] kernel/sysctl.c: do not override max_threads provided by userspace

[Pavel Machek]

Hi!

> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > 
> > > commit b0f53dbc4bc4c371f38b14c391095a3bb8a0bb40 upstream.
> > > 
> > > Partially revert 16db3d3f1170 ("kernel/sysctl.c: threads-max observe
> > > limits") because the patch is causing a regression to any workload which
> > > needs to override the auto-tuning of the limit provided by kernel.
> > > 
> > > set_max_threads is implementing a boot time guesstimate to provide a
> > > sensible limit of the concurrently running threads so that runaways will
> > > not deplete all the memory.  This is a good thing in general but there
> > > are workloads which might need to increase this limit for an application
> > > to run (reportedly WebSpher MQ is affected) and that is simply not
> > > possible after the mentioned change.  It is also very dubious to
> > > override an admin decision by an estimation that doesn't have any direct
> > > relation to correctness of the kernel operation.
> > > 
> > > Fix this by dropping set_max_threads from sysctl_max_threads so any
> > > value is accepted as long as it fits into MAX_THREADS which is important
> > > to check because allowing more threads could break internal robust futex
> > > restriction.  While at it, do not use MIN_THREADS as the lower boundary
> > > because it is also only a heuristic for automatic estimation and admin
> > > might have a good reason to stop new threads to be created even when
> > > below this limit.
> > 
> > Ok, why not, but I smell followup work could be done:
> > 
> > > @@ -2635,7 +2635,7 @@ int sysctl_max_threads(struct ctl_table
> > >  	if (ret || !write)
> > >  		return ret;
> > >  
> > > -	set_max_threads(threads);
> > > +	max_threads = threads;
> > >  
> > 
> > AFAICT set_max_threads can now become __init.
> 
> Yes. Care to send a patch?

I'm not usually hacking in that area. Could you do that?

> > Plus, I don't see any locking here, should this be WRITE_ONCE() at
> > minimum?
> 
> Why would that matter? Do you expect several root processes race to set
> the value?

Well, for example to warn humans that this code is accessing unlocked
variable. Second, as is, code is not valid C and compilers are
allowed to do strange stuff ("undefined behaviour"). Third, there are
concurency checkers that will not like this one.

Best regards,
								Pavel
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