linux-kernel - Re: [PATCH] [5/8] Fix logic error in 64bit memory hotadd

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Date:	Mon, 11 Feb 2008 13:46:25 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...e.de>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [5/8] Fix logic error in 64bit memory hotadd

* Andi Kleen <ak@...e.de> wrote:

> The memory hotadd code assumes that the pud always exists already, but 
> that might be not true. Allocate it if it isn't there.

ok, this seems an like an ancient memory-hotplug bug. Does anyone even 
use memory hotplug currently? Did you find this bug via review, or did 
it trigger in practice?

Also, your fix, while it solves a real bug we want to fix, is not quite 
right for upstream integration yet. I can see 3 immediate problems with 
it:

> +			if (!pud_present(*pud)) {
> +				pud = (pud_t *)get_zeroed_page(GFP_ATOMIC);

the GFP_ATOMIC here can fail.

The proper solution is to instead extend init_memory_mapping() with a 
gfp_t parameter and pass in GFP_ATOMIC from the early init code (where 
we must not schedule and where GFP_ATOMIC will succeed anyway), but do a 
GFP_KERNEL from arch_add_memory().

> +				if (!pud)
> +					panic("Out of memory");

the panic() here is very rude to the user in the hotplug usecase.

The proper solution is to extend init_memory_mapping() with a return 
value, and to check in the caller. arch_add_memory() obviously does not 
want to panic(), it wants to return -ENOMEM to mm/memory_hotplug.c.

and a small style nit while changing this code:

> +		} else
>  			pud = alloc_low_page(&pud_phys);

please add curly braces to the 'else' branch too. (we generally prefer 
symmetrical curly braces) Thanks,

	Ingo
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