linux-kernel - Re: [PATCH 1/4] compcache: xvmalloc memory allocator

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Message-ID: <4A92EBB4.1070101@vflare.org>
Date:	Tue, 25 Aug 2009 01:06:20 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-mm-cc@...top.org
Subject: Re: [PATCH 1/4] compcache: xvmalloc memory allocator

Hi Pekka,

On 08/24/2009 11:03 PM, Pekka Enberg wrote:

<snip>

> On Mon, Aug 24, 2009 at 7:37 AM, Nitin Gupta<ngupta@...are.org>  wrote:
>> +/**
>> + * xv_malloc - Allocate block of given size from pool.
>> + * @pool: pool to allocate from
>> + * @size: size of block to allocate
>> + * @pagenum: page no. that holds the object
>> + * @offset: location of object within pagenum
>> + *
>> + * On success,<pagenum, offset>  identifies block allocated
>> + * and 0 is returned. On failure,<pagenum, offset>  is set to
>> + * 0 and -ENOMEM is returned.
>> + *
>> + * Allocation requests with size>  XV_MAX_ALLOC_SIZE will fail.
>> + */
>> +int xv_malloc(struct xv_pool *pool, u32 size, u32 *pagenum, u32 *offset,
>> +                                                       gfp_t flags)

<snip>

>
> What's the purpose of passing PFNs around? There's quite a lot of PFN
> to struct page conversion going on because of it. Wouldn't it make
> more sense to return (and pass) a pointer to struct page instead?


PFNs are 32-bit on all archs while for 'struct page *', we require 32-bit or
64-bit depending on arch. ramzswap allocates a table entry <pagenum, offset>
corresponding to every swap slot. So, the size of table will unnecessarily
increase on 64-bit archs. Same is the argument for xvmalloc free list sizes.

Also, xvmalloc and ramzswap itself does PFN -> 'struct page *' conversion
only when freeing the page or to get a deferencable pointer.

Thanks,
Nitin

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