From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9CC74197A7D; Tue, 16 Jul 2024 12:58:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721134692; cv=none; b=qT5+AqoU6NKa6YQN2OZdLAocQ5HBl8lSVcOhe7TxYwCHve0meLR1lVl8hheCwkg5ZMptmfurNcGM0/sIcvhMmynvGaQ+Sf44Zg30eaBlNSdIKj+WamGULRzT1tGxLuI8a3F8CJRGplayBut9HH4hvNTzQjqF1zpQh8iIblZP9kY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721134692; c=relaxed/simple; bh=kz0CicEUTeN4A8dfwF0ec3vvlKnAWa8ICTwjgHDQoiw=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=SeLvSmvxNwiofynXZzWnby150La24MMcZXKp1qMTHYZ53MtibEILGPkm0r3SqJTYaPLLPQsbHfYBYanBeyg1PTG/5V6+XMPqqY0hV0qi7+PZsq1QnPXERn3wk05CA9rndR4BnkKsY0GjXberWy2AJsgPgv4mXAMBzP/Zmto1dis= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=45.249.212.189 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.19.88.105]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4WNfCB0gg4zQlZY; Tue, 16 Jul 2024 20:53:58 +0800 (CST) Received: from dggpemf200006.china.huawei.com (unknown [7.185.36.61]) by mail.maildlp.com (Postfix) with ESMTPS id 97B7F1403D1; Tue, 16 Jul 2024 20:58:00 +0800 (CST) Received: from [10.67.120.129] (10.67.120.129) by dggpemf200006.china.huawei.com (7.185.36.61) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Tue, 16 Jul 2024 20:58:00 +0800 Message-ID: <5a3b39b7-c183-4c73-bd9b-184db8b24f6a@huawei.com> Date: Tue, 16 Jul 2024 20:58:00 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v9 06/13] mm: page_frag: reuse existing space for 'size' and 'pfmemalloc' To: Alexander Duyck , Yunsheng Lin CC: , , , , , Andrew Morton , References: <20240625135216.47007-1-linyunsheng@huawei.com> <20240625135216.47007-7-linyunsheng@huawei.com> <12a8b9ddbcb2da8431f77c5ec952ccfb2a77b7ec.camel@gmail.com> <808be796-6333-c116-6ecb-95a39f7ad76e@huawei.com> <96b04ebb7f46d73482d5f71213bd800c8195f00d.camel@gmail.com> <5daed410-063b-4d86-b544-d1a85bd86375@huawei.com> <29e8ac53-f7da-4896-8121-2abc25ec2c95@gmail.com> <12ff13d9-1f3d-4c1b-a972-2efb6f247e31@gmail.com> Content-Language: en-US From: Yunsheng Lin In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemf200006.china.huawei.com (7.185.36.61) On 2024/7/16 1:55, Alexander Duyck wrote: > On Sat, Jul 13, 2024 at 9:52 PM Yunsheng Lin wrote: ... >> >> If the option 1 is not what you have in mind, it would be better to be >> more specific about what you have in mind. > > Option 1 was more or less what I had in mind. > >> If the option 1 is what you have in mind, it seems both option 1 and >> option 2 have the same semantics as my understanding, right? The >> question here seems to be what is your perfer option and why? >> >> I implemented both of them, and the option 1 seems to have a >> bigger generated asm size as below: >> ./scripts/bloat-o-meter vmlinux_non_neg vmlinux >> add/remove: 0/0 grow/shrink: 1/0 up/down: 37/0 (37) >> Function old new delta >> __page_frag_alloc_va_align 414 451 +37 > > My big complaint is that it seems option 2 is harder for people to > understand and more likely to not be done correctly. In some cases if > the performance difference is negligible it is better to go with the > more maintainable solution. Option 1 assuming nc->remaining as a negative value does not seems to make it a more maintainable solution than option 2. How about something like below if using a negative value to enable some optimization like LEA does not have a noticeable performance difference? struct page_frag_cache { /* encoded_va consists of the virtual address, pfmemalloc bit and order * of a page. */ unsigned long encoded_va; #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) && (BITS_PER_LONG <= 32) __u16 remaining; __u16 pagecnt_bias; #else __u32 remaining; __u32 pagecnt_bias; #endif }; void *__page_frag_alloc_va_align(struct page_frag_cache *nc, unsigned int fragsz, gfp_t gfp_mask, unsigned int align_mask) { unsigned int size = page_frag_cache_page_size(nc->encoded_va); unsigned int remaining; remaining = nc->remaining & align_mask; if (unlikely(remaining < fragsz)) { if (unlikely(fragsz > PAGE_SIZE)) { /* * The caller is trying to allocate a fragment * with fragsz > PAGE_SIZE but the cache isn't big * enough to satisfy the request, this may * happen in low memory conditions. * We don't release the cache page because * it could make memory pressure worse * so we simply return NULL here. */ return NULL; } if (!__page_frag_cache_refill(nc, gfp_mask)) return NULL; size = page_frag_cache_page_size(nc->encoded_va); remaining = size; } nc->pagecnt_bias--; nc->remaining = remaining - fragsz; return encoded_page_address(nc->encoded_va) + (size - remaining); }