linux-kernel - Re: [PATCH] mm: Fix comment for NODEMASK

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Date:   Tue, 21 Aug 2018 13:51:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Oscar Salvador <osalvador@...hadventures.net>
Cc:     Michal Hocko <mhocko@...nel.org>, tglx@...utronix.de,
        joe@...ches.com, arnd@...db.de, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH] mm: Fix comment for NODEMASK_ALLOC

On Tue, 21 Aug 2018 14:30:24 +0200 Oscar Salvador <osalvador@...hadventures.net> wrote:

> On Tue, Aug 21, 2018 at 02:17:34PM +0200, Michal Hocko wrote:
> > We do have CONFIG_NODES_SHIFT=10 in our SLES kernels for quite some
> > time (around SLE11-SP3 AFAICS).
> > 
> > Anyway, isn't NODES_ALLOC over engineered a bit? Does actually even do
> > larger than 1024 NUMA nodes? This would be 128B and from a quick glance
> > it seems that none of those functions are called in deep stacks. I
> > haven't gone through all of them but a patch which checks them all and
> > removes NODES_ALLOC would be quite nice IMHO.
> 
> No, maximum we can get is 1024 NUMA nodes.
> I checked this when writing another patch [1], and since having gone
> through all archs Kconfigs, CONFIG_NODES_SHIFT=10 is the limit.
> 
> NODEMASK_ALLOC gets only called from:
> 
> - unregister_mem_sect_under_nodes() (not anymore after [1])
> - __nr_hugepages_store_common (This does not seem to have a deep stack, we could use a normal nodemask_t)
> 
> But is also used for NODEMASK_SCRATCH (mainly used for mempolicy):
> 
> struct nodemask_scratch {
> 	nodemask_t	mask1;
> 	nodemask_t	mask2;
> };
> 
> that would make 256 bytes in case CONFIG_NODES_SHIFT=10.

And that sole site could use an open-coded kmalloc.