From: Yunsheng Lin <linyunsheng@huawei.com>
To: Alexander H Duyck <alexander.duyck@gmail.com>,
<davem@davemloft.net>, <kuba@kernel.org>, <pabeni@redhat.com>
Cc: <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>
Subject: Re: [PATCH net-next v2 09/15] mm: page_frag: reuse MSB of 'size' field for pfmemalloc
Date: Wed, 17 Apr 2024 21:19:48 +0800 [thread overview]
Message-ID: <8b7361c2-6f45-72e8-5aca-92e8a41a7e5e@huawei.com> (raw)
In-Reply-To: <37d012438d4850c3d7090e784e09088d02a2780c.camel@gmail.com>
On 2024/4/17 0:22, Alexander H Duyck wrote:
> On Mon, 2024-04-15 at 21:19 +0800, Yunsheng Lin wrote:
>> The '(PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)' case is for the
>> system with page size less than 32KB, which is 0x8000 bytes
>> requiring 16 bits space, change 'size' to 'size_mask' to avoid
>> using the MSB, and change 'pfmemalloc' field to reuse the that
>> MSB, so that we remove the orginal space needed by 'pfmemalloc'.
>>
>> For another case, the MSB of 'offset' is reused for 'pfmemalloc'.
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>> ---
>> include/linux/page_frag_cache.h | 13 ++++++++-----
>> mm/page_frag_cache.c | 5 +++--
>> 2 files changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
>> index fe5faa80b6c3..40a7d6da9ef0 100644
>> --- a/include/linux/page_frag_cache.h
>> +++ b/include/linux/page_frag_cache.h
>> @@ -12,15 +12,16 @@ struct page_frag_cache {
>> void *va;
>> #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>> __u16 offset;
>> - __u16 size;
>> + __u16 size_mask:15;
>> + __u16 pfmemalloc:1;
>> #else
>> - __u32 offset;
>> + __u32 offset:31;
>> + __u32 pfmemalloc:1;
>> #endif
>
> This seems like a really bad idea. Using a bit-field like this seems
> like a waste as it means that all the accesses now have to add
> additional operations to access either offset or size. It wasn't as if
> this is an oversized struct, or one that we are allocating a ton of. As
> such I am not sure why we need to optmize for size like this.
For the old 'struct page_frag' use case, there is one 'struct page_frag'
for every socket and task_struct, there may be tens of thousands of
them even in a 32 bit sysmem, which might mean a lof of memory even for
a single byte saving, see patch 13.
>
>> /* we maintain a pagecount bias, so that we dont dirty cache line
>> * containing page->_refcount every time we allocate a fragment.
>> */
>> unsigned int pagecnt_bias;
>> - bool pfmemalloc;
>> };
next prev parent reply other threads:[~2024-04-17 13:19 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20240415131941.51153-1-linyunsheng@huawei.com>
2024-04-15 13:19 ` [PATCH net-next v2 01/15] mm: page_frag: add a test module for page_frag Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 03/15] mm: page_frag: use free_unref_page() to free page fragment Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 04/15] mm: move the page fragment allocator from page_alloc into its own file Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 05/15] mm: page_frag: use initial zero offset for page_frag_alloc_align() Yunsheng Lin
2024-04-15 23:55 ` Alexander H Duyck
2024-04-16 13:11 ` Yunsheng Lin
2024-04-16 15:51 ` Alexander H Duyck
2024-04-17 13:17 ` Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 06/15] mm: page_frag: change page_frag_alloc_* API to accept align param Yunsheng Lin
2024-04-16 16:08 ` Alexander Duyck
2024-04-17 13:18 ` Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 07/15] mm: page_frag: add '_va' suffix to page_frag API Yunsheng Lin
2024-04-16 16:12 ` Alexander H Duyck
2024-04-17 13:18 ` Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 08/15] mm: page_frag: add two inline helper for " Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 09/15] mm: page_frag: reuse MSB of 'size' field for pfmemalloc Yunsheng Lin
2024-04-16 16:22 ` Alexander H Duyck
2024-04-17 13:19 ` Yunsheng Lin [this message]
2024-04-17 15:11 ` Alexander H Duyck
2024-04-18 9:39 ` Yunsheng Lin
2024-04-26 9:38 ` Yunsheng Lin
2024-04-29 14:49 ` Alexander Duyck
2024-04-30 12:05 ` Yunsheng Lin
2024-04-30 14:54 ` Alexander Duyck
2024-05-06 12:33 ` Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 10/15] mm: page_frag: reuse existing bit field of 'va' for pagecnt_bias Yunsheng Lin
2024-04-16 16:33 ` Alexander H Duyck
2024-04-17 13:23 ` Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 12/15] mm: page_frag: introduce prepare/commit API for page_frag Yunsheng Lin
2024-04-15 13:19 ` [PATCH net-next v2 14/15] mm: page_frag: update documentation " Yunsheng Lin
2024-04-16 6:13 ` Bagas Sanjaya
2024-04-16 13:11 ` Yunsheng Lin
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