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From: Ingo Molnar <mingo@elte.hu>
To: Andi Kleen <ak@suse.de>
Cc: tglx@linutronix.de, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] [5/8] Fix logic error in 64bit memory hotadd
Date: Mon, 11 Feb 2008 13:46:25 +0100	[thread overview]
Message-ID: <20080211124625.GB23733@elte.hu> (raw)
In-Reply-To: <20080211093433.DE4961B41CE@basil.firstfloor.org>


* Andi Kleen <ak@suse.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

  reply	other threads:[~2008-02-11 12:46 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-11  9:34 [PATCH] [0/8] Various kernel mapping bug fixes Andi Kleen
2008-02-11  9:34 ` [PATCH] [1/8] CPA: Fix gbpages support in try_preserve_lage_page Andi Kleen
2008-02-11  9:45   ` Thomas Gleixner
2008-02-11 10:12     ` Ingo Molnar
2008-02-11 11:01       ` Andi Kleen
2008-02-11  9:34 ` [PATCH] [2/8] CPA: Flush the caches when setting pages not present Andi Kleen
2008-02-11 11:00   ` Ingo Molnar
2008-02-11 12:26     ` Andi Kleen
2008-02-11  9:34 ` [PATCH] [3/8] CPA: Test the correct mapping alias on x86-64 Andi Kleen
2008-02-11 11:49   ` Ingo Molnar
2008-02-11  9:34 ` [PATCH] [4/8] CPA: Fix set_memory_x for ioremap Andi Kleen
2008-02-11 12:27   ` Ingo Molnar
2008-02-11 12:45     ` Andi Kleen
2008-02-11  9:34 ` [PATCH] [5/8] Fix logic error in 64bit memory hotadd Andi Kleen
2008-02-11 12:46   ` Ingo Molnar [this message]
2008-02-11 13:05     ` Andi Kleen
2008-02-11 13:33       ` Ingo Molnar
2008-02-11 13:44         ` Andi Kleen
2008-02-12 10:35       ` Yasunori Goto
2008-02-12 11:20         ` Andi Kleen
2008-02-11  9:34 ` [PATCH] [6/8] Account overlapped mappings in end_pfn_map Andi Kleen
2008-02-11 13:08   ` Ingo Molnar
2008-02-11 13:27     ` Andi Kleen
2008-02-11 13:55       ` Ingo Molnar
2008-02-11 14:16       ` Peter Zijlstra
2008-02-11 14:24         ` Andi Kleen
2008-02-11 14:41           ` Sam Ravnborg
2008-02-11 15:12   ` Arjan van de Ven
2008-02-11  9:34 ` [PATCH] [7/8] Implement true end_pfn_mapped for 32bit Andi Kleen
2008-02-12 19:39   ` Thomas Gleixner
2008-02-12 19:49     ` Andi Kleen
2008-02-12 20:25       ` Thomas Gleixner
2008-02-11  9:34 ` [PATCH] [8/8] RFC: Fix some EFI problems Andi Kleen
2008-02-12 20:04   ` Thomas Gleixner
2008-02-12 20:23     ` Andi Kleen
2008-02-12 20:48       ` Thomas Gleixner
2008-02-13 11:05         ` Andi Kleen

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