linux-kernel - Re: [PATCH net-next v13 07/14] mm: page_frag: reuse existing space for 'size' and 'pfmemalloc'

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Message-ID: <a036abc3-76a0-450c-afea-2db3c10f0ed5@huawei.com>
Date: Thu, 15 Aug 2024 11:10:49 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Alexander H Duyck <alexander.duyck@...il.com>, <davem@...emloft.net>,
	<kuba@...nel.org>, <pabeni@...hat.com>
CC: <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Andrew Morton
	<akpm@...ux-foundation.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH net-next v13 07/14] mm: page_frag: reuse existing space
 for 'size' and 'pfmemalloc'

On 2024/8/15 0:13, Alexander H Duyck wrote:
> On Thu, 2024-08-08 at 20:37 +0800, Yunsheng Lin wrote:
>> Currently there is one 'struct page_frag' for every 'struct
>> sock' and 'struct task_struct', we are about to replace the
>> 'struct page_frag' with 'struct page_frag_cache' for them.
>> Before begin the replacing, we need to ensure the size of
>> 'struct page_frag_cache' is not bigger than the size of
>> 'struct page_frag', as there may be tens of thousands of
>> 'struct sock' and 'struct task_struct' instances in the
>> system.
>>
>> By or'ing the page order & pfmemalloc with lower bits of
>> 'va' instead of using 'u16' or 'u32' for page size and 'u8'
>> for pfmemalloc, we are able to avoid 3 or 5 bytes space waste.
>> And page address & pfmemalloc & order is unchanged for the
>> same page in the same 'page_frag_cache' instance, it makes
>> sense to fit them together.
>>
>> After this patch, the size of 'struct page_frag_cache' should be
>> the same as the size of 'struct page_frag'.
>>
>> CC: Alexander Duyck <alexander.duyck@...il.com>
>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
>> ---
>>  include/linux/mm_types_task.h   | 16 +++++-----
>>  include/linux/page_frag_cache.h | 52 +++++++++++++++++++++++++++++++--
>>  mm/page_frag_cache.c            | 49 +++++++++++++++++--------------
>>  3 files changed, 85 insertions(+), 32 deletions(-)
>>
>> diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
>> index b1c54b2b9308..f2610112a642 100644
>> --- a/include/linux/mm_types_task.h
>> +++ b/include/linux/mm_types_task.h
>> @@ -50,18 +50,18 @@ struct page_frag {
>>  #define PAGE_FRAG_CACHE_MAX_SIZE	__ALIGN_MASK(32768, ~PAGE_MASK)
>>  #define PAGE_FRAG_CACHE_MAX_ORDER	get_order(PAGE_FRAG_CACHE_MAX_SIZE)
>>  struct page_frag_cache {
>> -	void *va;
>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> +	/* encoded_va consists of the virtual address, pfmemalloc bit and order
>> +	 * of a page.
>> +	 */
>> +	unsigned long encoded_va;
>> +
> 
> Rather than calling this an "encoded_va" we might want to call this an
> "encoded_page" as that would be closer to what we are actually working
> with. We are just using the virtual address as the page pointer instead
> of the page struct itself since we need quicker access to the virtual
> address than we do the page struct.

Calling it "encoded_page" seems confusing enough when calling virt_to_page()
with "encoded_page" when virt_to_page() is expecting a 'va', no?

>