Re: [patch v2 1/3] mm: remove GFP_THISNODE

[David Rientjes]

On Fri, 27 Feb 2015, Christoph Lameter wrote:

> > +/*
> > + * Construct gfp mask to allocate from a specific node but do not invoke reclaim
> > + * or warn about failures.
> > + */
> 
> We should be triggering reclaim from slab allocations. Why would we not do
> this?
> 
> Otherwise we will be going uselessly off node for slab allocations.
> 
> > +static inline gfp_t gfp_exact_node(gfp_t flags)
> > +{
> > +	return (flags | __GFP_THISNODE | __GFP_NOWARN) & ~__GFP_WAIT;
> > +}
> >  #endif
> 
> Reclaim needs to be triggered. In particular zone reclaim was made to be
> triggered from slab allocations to create more room if needed.
> 

This illustrates the precise need for a patch like this that removes 
GFP_THISNODE entirely: notice there's no functional change with this 
patch.

GFP_THISNODE is __GFP_THISNODE | __GFP_NOWARN | __GFP_NORETRY.

The calls to ____cache_alloc_node() and cache_grow() modified by this 
patch in mm/slab.c that pass GFP_THISNODE get caught in the page allocator 
slowpath by this:

	if (IS_ENABLED(CONFIG_NUMA) &&
	    (gfp_mask & GFP_THISNODE) == GFP_THISNODE)
		goto nopage;

with today's kernel.  In fact, there is no way for the slab allocator to 
currently allocate exactly on one node, allow reclaim, and avoid looping 
forever while suppressing the page allocation failure warning.  The reason 
is because of how GFP_THISNODE is defined above.

With this patch, it would be possible to modify gfp_exact_node() so that 
instead of doing

	return (flags | __GFP_THISNODE | __GFP_NOWARN) & ~__GFP_WAIT;

which has no functional change from today, it could be

	return flags | __GFP_THISNODE | __GFP_NOWARN | __GFP_NORETRY;

so that we _can_ do reclaim for that node and avoid looping forever when 
the allocation fails.  These three flags are the exact same bits set in 
today's GFP_THISNODE and it is, I agree, what the slab allocator really 
wants to do in cache_grow().  But the conditional above is what 
short-circuits such an allocation and needs to be removed, which is what 
this patch does.

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