linux-kernel - Re: [Patch v2 2/2] mm/page_alloc.c: define node

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Message-ID: <b1b41da1-2ced-8bb8-7162-e5c820543244@nvidia.com>
Date:   Sat, 28 Mar 2020 18:30:00 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Wei Yang <richard.weiyang@...il.com>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <jgg@...pe.ca>, <david@...hat.com>
Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero

On 3/27/20 7:56 PM, Wei Yang wrote:
...
>> Further note: On my current testing .config, I've got MAX_NUMNODES set to 64, which makes
>> 256 bytes required for node_order array. 256 bytes on a 16KB stack is a little bit above
>> my mental watermark for "that's too much in today's kernels".
>>
> 
> Thanks for your explanation. I would keep this in mind.
> 
> Now I have one more question, hope it won't sound silly. (16KB / 256) = 64,
> this means if each function call takes 256 space on stack, the max call depth
> is 64. So how deep a kernel function call would be? or expected to be?
>

64 is quite a bit deeper call depth than we expect to see. So 256 bytes on the stack
is not completely indefensible, but it's getting close. But worse, that's just an
example based on my .config choices. And (as Baoquan just pointed out) it can be much
bigger. (And the .config variable is an exponent, not linear, so it gets exciting fast.)
In fact, you could overrun the stack directly, with say CONFIG_NODES_SHIFT = 14.

  
> Also because of the limit space on stack, recursive function is not welcome in
> kernel neither. Am I right?
> 
Yes, that is correct.

thanks,
-- 
John Hubbard
NVIDIA