Message-ID: <543548C3.7030003@oracle.com>
Date: Wed, 08 Oct 2014 10:22:59 -0400
From: Sasha Levin <sasha.levin@oracle.com>
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To: Dave Hansen <dave.hansen@intel.com>, akpm@linux-foundation.org
CC: linux-kernel@vger.kernel.org, linux-mm@kvack.org, hughd@google.com,
        mgorman@suse.de, Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH 5/5] mm: poison page struct
References: <1412041639-23617-1-git-send-email-sasha.levin@oracle.com> <1412041639-23617-6-git-send-email-sasha.levin@oracle.com> <5434630C.3070006@intel.com>
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On 10/07/2014 06:02 PM, Dave Hansen wrote:
> On 09/29/2014 06:47 PM, Sasha Levin wrote:
>>  struct page {
>> +#ifdef CONFIG_DEBUG_VM_POISON
>> +	u32 poison_start;
>> +#endif
>>  	/* First double word block */
>>  	unsigned long flags;		/* Atomic flags, some possibly
>>  					 * updated asynchronously */
>> @@ -196,6 +199,9 @@ struct page {
>>  #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
>>  	int _last_cpupid;
>>  #endif
>> +#ifdef CONFIG_DEBUG_VM_POISON
>> +	u32 poison_end;
>> +#endif
>>  }
> 
> Does this break slub's __cmpxchg_double_slab trick?  I thought it
> required page->freelist and page->counters to be doubleword-aligned.

I'll probably have to switch it to 8 bytes anyways to make it work with
kasan. This should take care of the slub optimization as well.

> It's not like we really require this optimization when we're debugging,
> but trying to use it will unnecessarily slow things down.
> 
> FWIW, if you're looking to trim down the number of lines of code, you
> could certainly play some macro tricks and #ifdef tricks.
> 
> struct vm_poison {
> #ifdef CONFIG_DEBUG_VM_POISON
> 	u32 val;
> #endif	
> };
> 
> Then, instead of #ifdefs in each structure, you do:
> 
> struct page {
> 	struct vm_poison poison_start;
> 	... other gunk
> 	struct vm_poison poison_end;
> };

Agreed, I'll reword that in the next version.


Thanks,
Sasha

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