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

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Date:   Tue, 21 Aug 2018 14:30:24 +0200
From:   Oscar Salvador <osalvador@...hadventures.net>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.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, 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.
I am not familiar with mempolicy code, I am not sure if we can do without that and
figure out another way to achieve the same.

[1] https://patchwork.kernel.org/patch/10566673/#22179663 

-- 
Oscar Salvador
SUSE L3