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From: Andi Kleen <ak@suse.de>
To: Ingo Molnar <mingo@elte.hu>
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 14:05:04 +0100	[thread overview]
Message-ID: <200802111405.04423.ak@suse.de> (raw)
In-Reply-To: <20080211124625.GB23733@elte.hu>

On Monday 11 February 2008 13:46:25 Ingo Molnar wrote:
> 
> * 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.

Yes.

> Does anyone even  
> use memory hotplug currently? 

I don't know.

> Did you find this bug via review, or did  
> it trigger in practice?

Review.

> 
> 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 memory hotplug code already uses GFP_ATOMIC elsewhere (spp_getpage) 
 
> 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().

The existing code already does GFP_ATOMIC. I admit I haven't double
checked why it does that (didn't read the complete path) but I assume
it takes a spin lock somewhere.

If there is no lock doing a general clean up of all of them would probably
make sense.  But it would be orthogonal to my patch and I don't think
it's needed to fix this concrete bug.

The gfp argument is not needed though because this
case can be already distingushed by checking after_bootmem.

> 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.

The existing code already panics elsewhere (spp_getpage); i just copied
that.

So in summary the panic&GFP_ATOMIC use are not good (I agree), but it's
not worse than what was in there before.

-Andi

  reply	other threads:[~2008-02-11 13:05 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
2008-02-11 13:05     ` Andi Kleen [this message]
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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