Re: [PATCH] mm: Check for SIGKILL inside dup_mmap() loop.

[Michal Hocko <mhocko@kernel.org>]

On Tue 03-04-18 05:14:14, Matthew Wilcox wrote:
> On Fri, Mar 30, 2018 at 07:34:59PM +0900, Tetsuo Handa wrote:
> > Maybe we can make "give up by default upon SIGKILL" and let callers
> > explicitly say "do not give up upon SIGKILL".
> 
> I really strongly disapprove of this patch.  This GFP flag will be abused
> like every other GFP flag.
> 
> > +++ b/mm/page_alloc.c
> > @@ -4183,6 +4183,13 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
> >  	if (current->flags & PF_MEMALLOC)
> >  		goto nopage;
> >  
> > +	/* Can give up if caller is willing to give up upon fatal signals */
> > +	if (fatal_signal_pending(current) &&
> > +	    !(gfp_mask & (__GFP_UNKILLABLE | __GFP_NOFAIL))) {
> > +		gfp_mask |= __GFP_NOWARN;
> > +		goto nopage;
> > +	}
> > +
> >  	/* Try direct reclaim and then allocating */
> 
> This part is superficially tempting, although without the UNKILLABLE.  ie:
> 
> +	if (fatal_signal_pending(current) && !(gfp_mask & __GFP_NOFAIL)) {
> +		gfp_mask |= __GFP_NOWARN;
> +		goto nopage;
> +	}
> 
> It makes some sense to me to prevent tasks with a fatal signal pending
> from being able to trigger reclaim.  But I'm worried about what memory
> allocation failures it might trigger on paths that aren't accustomed to
> seeing failures.

Please be aware that we _do_ allocate in the exit path. I have a strong
suspicion that even while fatal signal is pending. Do we really want
fail those really easily.
-- 
Michal Hocko
SUSE Labs