linux-kernel - Re: [RFC][PATCH] Manage jbd allocations from its own slabs

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Date:	Thu, 24 Aug 2006 12:11:25 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	Christoph Hellwig <hch@...radead.org>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, akpm@...l.org,
	lkml <linux-kernel@...r.kernel.org>,
	ext2-devel <Ext2-devel@...ts.sourceforge.net>
Subject: Re: [RFC][PATCH] Manage jbd allocations from its own slabs

Christoph Hellwig wrote:
> On Wed, Aug 23, 2006 at 04:08:15PM -0700, Badari Pulavarty wrote:
>   
>> Hi,
>>
>> Here is the fix to "bh: Ensure bh fits within a page" problem
>> caused by JBD.
>>
>> BTW, I realized that this problem can happen only with 1k, 2k
>> filesystems - as 4k, 8k allocations disable slab debug 
>> automatically. But for completeness, I created slabs for those
>> also.
>>
>> What do you think ? I ran basic tests and things are fine.
>>     
>
> Why can't you just use alloc_page?  I bet the whole slab overhead
> eats more memory than what's wasted when using alloc_pages.  Especially
> as the typical usecase is a 4k blocks filesystem with 4k pagesize
> where the overhead of alloc_page is non-existant.
>   

Yes. That was what proposed earlier. But for 1k, 2k allocations we end 
up wasting whole page.
Isn't it ? Thats why I created right sized slabs and disable slab-debug. 
I guess, I can do this
only for 1k, 2k filesystems and directly use alloc_page() for 4k and 8k 
- but that would make
code ugly and also it doesn't handle cases for bigger base pagesize 
systems (64k power).

Thanks,
Badari


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