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Mon, 8 Apr 2024 21:39:06 +0800 Subject: Re: [PATCH net-next v1 02/12] mm: page_frag: use initial zero offset for page_frag_alloc_align() To: Alexander H Duyck , , , CC: , , Andrew Morton , References: <20240407130850.19625-1-linyunsheng@huawei.com> <20240407130850.19625-3-linyunsheng@huawei.com> <43d99616cd4a2a6fce6a6b97f73d08ebc5361a61.camel@gmail.com> From: Yunsheng Lin Message-ID: Date: Mon, 8 Apr 2024 21:39:06 +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: <43d99616cd4a2a6fce6a6b97f73d08ebc5361a61.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: dggems701-chm.china.huawei.com (10.3.19.178) To dggpemm500005.china.huawei.com (7.185.36.74) X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 56F9540003 X-Stat-Signature: 13ukxdmdh348kizuerkecswq6qy6bxn5 X-HE-Tag: 1712583549-70365 X-HE-Meta: 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 AL8fVYx+ zN40FGRtG+bHKiK1qGQVp/MmlAX6jrtCUrViZJK6zu/6MpF58taaOz1QBH/BQUxbjQ8Yw4M31z4rWkw/zAc6Z/PIOH9WF+xcnyQpY2WWgoimBz8bFHSYpvvlhap3frTGh6vTPdNiqBxD+2qv3hL8OVOREe9V4nCvwcO7PhsG0COPb5K2uoUEZRyg/2BbRyUMSan6cFQPweaEMPHUp/McZr262NLbnuvJnU1fh3m33GXhgEN33C8PE4zGcNIBuGUMxuaXOqFegGLS924JTrRUbuCVX94RRVvQCcUoCv4dlGLkEPv5wiF+TCLANTwr2i7eVGGK1FaCUZyUTp0BYrWah6hkPxD+ftNdJ0daNj4xMsNAFFXE= 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 2024/4/8 1:52, Alexander H Duyck wrote: > On Sun, 2024-04-07 at 21:08 +0800, Yunsheng Lin wrote: >> We are above to use page_frag_alloc_*() API to not just >> allocate memory for skb->data, but also use them to do >> the memory allocation for skb frag too. Currently the >> implementation of page_frag in mm subsystem is running >> the offset as a countdown rather than count-up value, >> there may have several advantages to that as mentioned >> in [1], but it may have some disadvantages, for example, >> it may disable skb frag coaleasing and more correct cache >> prefetching >> >> We have a trade-off to make in order to have a unified >> implementation and API for page_frag, so use a initial zero >> offset in this patch, and the following patch will try to >> make some optimization to aovid the disadvantages as much >> as possible. >> >> 1. https://lore.kernel.org/all/f4abe71b3439b39d17a6fb2d410180f367cadf5c.camel@gmail.com/ >> >> CC: Alexander Duyck >> Signed-off-by: Yunsheng Lin >> --- >> mm/page_frag_cache.c | 31 ++++++++++++++----------------- >> 1 file changed, 14 insertions(+), 17 deletions(-) >> >> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c >> index a0f90ba25200..3e3e88d9af90 100644 >> --- a/mm/page_frag_cache.c >> +++ b/mm/page_frag_cache.c >> @@ -67,9 +67,8 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc, >> unsigned int fragsz, gfp_t gfp_mask, >> unsigned int align_mask) >> { >> - unsigned int size = PAGE_SIZE; >> + unsigned int size, offset; >> struct page *page; >> - int offset; >> >> if (unlikely(!nc->va)) { >> refill: >> @@ -77,10 +76,6 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc, >> if (!page) >> return NULL; >> >> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) >> - /* if size can vary use size else just use PAGE_SIZE */ >> - size = nc->size; >> -#endif >> /* Even if we own the page, we do not use atomic_set(). >> * This would break get_page_unless_zero() users. >> */ >> @@ -89,11 +84,18 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc, >> /* reset page count bias and offset to start of new frag */ >> nc->pfmemalloc = page_is_pfmemalloc(page); >> nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; >> - nc->offset = size; >> + nc->offset = 0; >> } >> >> - offset = nc->offset - fragsz; >> - if (unlikely(offset < 0)) { >> +#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) >> + /* if size can vary use size else just use PAGE_SIZE */ >> + size = nc->size; >> +#else >> + size = PAGE_SIZE; >> +#endif >> + >> + offset = ALIGN(nc->offset, -align_mask); >> + if (unlikely(offset + fragsz > size)) { > > Rather than using "ALIGN" with a negative value it would probably make > more sense to use __ALIGN_KERNEL_MASK with ~align_mask. I am not sure > how well the compiler sorts out the use of negatives to flip values > that are then converted to masks with the "(a) - 1". The next patch will remove the '-' in '-align_mask' as the 'ALIGN' operation is done in the inline helper. I am not sure that matter much to use __ALIGN_KERNEL_MASK with ~align_mask? >