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Mon, 6 May 2024 20:33:58 +0800 Subject: Re: [PATCH net-next v2 09/15] mm: page_frag: reuse MSB of 'size' field for pfmemalloc To: Alexander Duyck CC: , , , , , Andrew Morton , References: <20240415131941.51153-1-linyunsheng@huawei.com> <20240415131941.51153-10-linyunsheng@huawei.com> <37d012438d4850c3d7090e784e09088d02a2780c.camel@gmail.com> <8b7361c2-6f45-72e8-5aca-92e8a41a7e5e@huawei.com> <17066b6a4f941eea3ef567767450b311096da22b.camel@gmail.com> From: Yunsheng Lin Message-ID: Date: Mon, 6 May 2024 20:33:58 +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: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To dggpemm500005.china.huawei.com (7.185.36.74) X-Stat-Signature: mcfjxpgt7sh1fgunogthtkt7scdhn94t X-Rspamd-Queue-Id: 859AE100018 X-Rspamd-Server: rspam10 X-Rspam-User: X-HE-Tag: 1714998842-666074 X-HE-Meta: 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 hfYGLzrf mJl0BzCTdUvTp8N6B3lamovNMrmYiAtxVLEHnwoG11wVIBN8Z9VZ7ItsgiNrPtl3upI/gr2sC3rg0BwkmZifom5VaoVpzFDiod5Vd3j3yWxTfbRABIOHGO2MQE/n14EY+5quqwIo/9LID6X14WXSeBh0o3sYAOCAUSqnWtnidZGFRCFTSQ7kVIxv/GpPOom0R/GnGRm6oBTbdIQqd7nWrfIoRS4xheR4yzecyZuEb+SIasz2eZw4B4BMjx38BYweVTeX2L/Tdlrn19HpnQsfVR5GDb7jxe5ZLt4e2gQyKNnLt89TEYFFrStqFFIdRknpRoNhHnMl0PFWY82CrArkeAsPzLPLo7zQeO+Et0QKWCvSjhcE= 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 2024/4/30 22:54, Alexander Duyck wrote: > On Tue, Apr 30, 2024 at 5:06 AM Yunsheng Lin wrote: >> >> On 2024/4/29 22:49, Alexander Duyck wrote: >> >> ... >> >>>>> >>>> >>>> After considering a few different layouts for 'struct page_frag_cache', >>>> it seems the below is more optimized: >>>> >>>> struct page_frag_cache { >>>> /* page address & pfmemalloc & order */ >>>> void *va; >>> >>> I see. So basically just pack the much smaller bitfields in here. >>> >>>> >>>> #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) && (BITS_PER_LONG <= 32) >>>> u16 pagecnt_bias; >>>> u16 size; >>>> #else >>>> u32 pagecnt_bias; >>>> u32 size; >>>> #endif >>>> } >>>> >>>> The lower bits of 'va' is or'ed with the page order & pfmemalloc instead >>>> of offset or pagecnt_bias, so that we don't have to add more checking >>>> for handling the problem of not having enough space for offset or >>>> pagecnt_bias for PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE and 32 bits system. >>>> And page address & pfmemalloc & order is unchanged for the same page >>>> in the same 'page_frag_cache' instance, it makes sense to fit them >>>> together. >>>> >>>> Also, it seems it is better to replace 'offset' with 'size', which indicates >>>> the remaining size for the cache in a 'page_frag_cache' instance, and we >>>> might be able to do a single 'size >= fragsz' checking for the case of cache >>>> being enough, which should be the fast path if we ensure size is zoro when >>>> 'va' == NULL. >>> >>> I'm not sure the rename to size is called for as it is going to be >>> confusing. Maybe something like "remaining"? >> >> Yes, using 'size' for that is a bit confusing. >> Beside the above 'remaining', by googling, it seems we may have below >> options too: >> 'residual','unused', 'surplus' >> >>> >>>> Something like below: >>>> >>>> #define PAGE_FRAG_CACHE_ORDER_MASK GENMASK(1, 0) >>>> #define PAGE_FRAG_CACHE_PFMEMALLOC_BIT BIT(2) >>> >>> The only downside is that it is ossifying things so that we can only >> >> There is 12 bits that is always useful, we can always extend ORDER_MASK >> to more bits if lager order number is needed. >> >>> ever do order 3 as the maximum cache size. It might be better to do a >>> full 8 bytes as on x86 it would just mean accessing the low end of a >>> 16b register. Then you can have pfmemalloc at bit 8. >> >> I am not sure I understand the above as it seems we may have below option: >> 1. Use somthing like the above ORDER_MASK and PFMEMALLOC_BIT. >> 2. Use bitfield as something like below: >> >> unsigned long va:20;---or 52 for 64bit system >> unsigned long pfmemalloc:1 >> unsigned long order:11; >> >> Or is there a better idea in your mind? > > All I was suggesting was to make the ORDER_MASK 8 bits. Doing that the > compiler can just store VA in a register such as RCX and instead of > having to bother with a mask it could then just use CL to access the > order. As far as the flag bits such as pfmemalloc the 4 bits starting > at 8 would make the most sense since it doesn't naturally align to > anything and would have to be masked anyway. Ok. > > Using a bitfield like you suggest would be problematic as the compiler > would assume a shift is needed so you would have to add a shift to > your code to offset it for accessing VA. > >>> >>>> struct page_frag_cache { >>>> /* page address & pfmemalloc & order */ >>>> void *va; >>>> >>> >>> When you start combining things like this we normally would convert va >>> to an unsigned long. >> >> Ack. It seems it makes more sense to convert va to something like 'struct encoded_va' mirroring 'struct encoded_page' in below: https://elixir.bootlin.com/linux/v6.7-rc8/source/include/linux/mm_types.h#L222 >> >>> >>>> #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) && (BITS_PER_LONG <= 32) >>>> u16 pagecnt_bias; >>>> u16 size; >>>> #else >>>> u32 pagecnt_bias; >>>> u32 size; >>>> #endif >>>> }; >>>> >>>>