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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 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-Originating-IP: [10.67.120.129] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemf200006.china.huawei.com (7.185.36.61) X-Rspamd-Queue-Id: 607C5180010 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 3raw8yuxkxkt7j8ufsywtdcq5co31aqj X-HE-Tag: 1721134685-833642 X-HE-Meta: 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 ZMIwKOOT Th+nYYw3uJ5VxsMaoiDHsNUzVB18CI86UDfGWU2oUI1xRLWe2jvbS9E2PZnJDJ5esp4HxJ8782/gugVi8hAGO8u0VW/S7l4EGMyKnLQ9brZ8iadvTJ+1nFQVQbOS6jbsLV80MavPL/CHRtGxDQh1CYJxk5N3j3OgUotBs+cAMXiLBdZG2jiRQ0G4UXVCKmjpF8/4adTu7hNibtWfcz8tVAwvyh5pOBBxCqY4ku0MZ71LqltkWbXaktkgqwEh2guAW5o3bzHTPfj8bEtJSRDdksg0D21F/wnc2qyjVu541VrgqCm/sz5VSbq0G3FkbKNVdYULjXGFA/lOO+iUWlFXo9o2wPA== 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 2024/7/16 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); }