linux-kernel - Re: [PATCH 5/5] mm: poison page struct

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Date:	Wed, 08 Oct 2014 10:22:59 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Dave Hansen <dave.hansen@...el.com>, akpm@...ux-foundation.org
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	mgorman@...e.de, Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 5/5] mm: poison page struct

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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