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

[Michal Hocko]

On Thu 15-08-19 15:15:09, Andrew Morton wrote:
> On Thu, 15 Aug 2019 10:44:29 +0200 Michal Hocko <mhocko at kernel.org> wrote:
> 
> > > I continue to struggle with this.  It introduces a new kernel state
> > > "running preemptibly but must not call schedule()".  How does this make
> > > any sense?
> > > 
> > > Perhaps a much, much more detailed description of the oom_reaper
> > > situation would help out.
> >  
> > The primary point here is that there is a demand of non blockable mmu
> > notifiers to be called when the oom reaper tears down the address space.
> > As the oom reaper is the primary guarantee of the oom handling forward
> > progress it cannot be blocked on anything that might depend on blockable
> > memory allocations. These are not really easy to track because they
> > might be indirect - e.g. notifier blocks on a lock which other context
> > holds while allocating memory or waiting for a flusher that needs memory
> > to perform its work. If such a blocking state happens that we can end up
> > in a silent hang with an unusable machine.
> > Now we hope for reasonable implementations of mmu notifiers (strong
> > words I know ;) and this should be relatively simple and effective catch
> > all tool to detect something suspicious is going on.
> > 
> > Does that make the situation more clear?
> 
> Yes, thanks, much.  Maybe a code comment along the lines of
> 
>   This is on behalf of the oom reaper, specifically when it is
>   calling the mmu notifiers.  The problem is that if the notifier were
>   to block on, for example, mutex_lock() and if the process which holds
>   that mutex were to perform a sleeping memory allocation, the oom
>   reaper is now blocked on completion of that memory allocation.

    reaper is now blocked on completion of that memory allocation
    and cannot provide the guarantee of the OOM forward progress.

OK. 
 
> btw, do we need task_struct.non_block_count?  Perhaps the oom reaper
> thread could set a new PF_NONBLOCK (which would be more general than
> PF_OOM_REAPER).  If we run out of PF_ flags, use (current == oom_reaper_th).

Well, I do not have a strong opinion here. A simple check for the value
seems to be trivial. There are quite some holes in task_struct to hide
this counter without increasing the size.
-- 
Michal Hocko
SUSE Labs