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Wed, 17 Apr 2024 21:19:48 +0800 Subject: Re: [PATCH net-next v2 09/15] mm: page_frag: reuse MSB of 'size' field for pfmemalloc To: Alexander H Duyck , , , CC: , , Andrew Morton , References: <20240415131941.51153-1-linyunsheng@huawei.com> <20240415131941.51153-10-linyunsheng@huawei.com> <37d012438d4850c3d7090e784e09088d02a2780c.camel@gmail.com> From: Yunsheng Lin Message-ID: <8b7361c2-6f45-72e8-5aca-92e8a41a7e5e@huawei.com> Date: Wed, 17 Apr 2024 21:19:48 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <37d012438d4850c3d7090e784e09088d02a2780c.camel@gmail.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To dggpemm500005.china.huawei.com (7.185.36.74) X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: BC11A1A0016 X-Stat-Signature: 6qzy36dxpff3zhyrhaqhkaphxtankbpx X-HE-Tag: 1713359991-531050 X-HE-Meta: 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 DdtopPpZ 8z0nelT+lRKkZ6JABSt8bd5duL4I+xwWJ2GW3m63RaA82BY03j3/CGnAqDScYdX6tup4+XmzPuUP4AmZj2h411zHuhnFyAPnxjAKJUEhAqD3qXlY9Dkay9GX2a1xLunwossXOltKXiy6mvJFI5dwQitBIYYoj2njxDe0Mpr78CQon8mLmcuFpa7lUKycTHrnXn+11ujxjfgLDKZE08e30LQDR5v9a2kVvbIbPpsNqUZ3/z4Cfwk2apIcopzaki+/y8ZFEJDO0/KWdf/lpIsfQlaQQZOS5aecaQoeB4xj0vlSW0r0CbSlsO5cK1g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >> --- >> 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; >> };