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Sat, 09 Jan 2021 13:22:53 -0800 (PST) To: Al Viro Cc: linux-fsdevel@vger.kernel.org, Jens Axboe , linux-kernel@vger.kernel.org References: <6e795064-fdbd-d354-4b01-a4f7409debf5@gmail.com> <54cd4d1b-d7ec-a74c-8be0-e48780609d56@gmail.com> <20210109170359.GT3579531@ZenIV.linux.org.uk> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH] iov_iter: optimise iter type checking Message-ID: <93388ab0-abbd-a680-66e0-e1ba77981479@gmail.com> Date: Sat, 9 Jan 2021 21:19:21 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <20210109170359.GT3579531@ZenIV.linux.org.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 09/01/2021 17:03, Al Viro wrote: > On Sat, Jan 09, 2021 at 04:09:08PM +0000, Pavel Begunkov wrote: >> On 06/12/2020 16:01, Pavel Begunkov wrote: >>> On 21/11/2020 14:37, Pavel Begunkov wrote: >>>> The problem here is that iov_iter_is_*() helpers check types for >>>> equality, but all iterate_* helpers do bitwise ands. This confuses >>>> compilers, so even if some cases were handled separately with >>>> iov_iter_is_*(), corresponding ifs in iterate*() right after are not >>>> eliminated. >>>> >>>> E.g. iov_iter_npages() first handles discards, but iterate_all_kinds() >>>> still checks for discard iter type and generates unreachable code down >>>> the line. >>> >>> Ping. This one should be pretty simple >> >> Ping please. Any doubts about this patch? > > Sorry, had been buried in other crap. I'm really not fond of the > bitmap use; if anything, I would rather turn iterate_and_advance() et.al. > into switches... > > How about moving the READ/WRITE part into MSB? Checking is just as fast > (if not faster - check for sign vs. checking bit 0). And turn the > types into straight (dense) enum. Didn't realise that approach before, sounds good. Most of it will be replaced with sign jcc, and the rest will be (t >> 31) or movcc, so it should not be of concern. type_mask = 255; iov_iter_type(i) { return i->type & ~type_mask; } I hope this stuff won't add much, because the original patch completely optimises this "&" out. I guess it'll turn into extra xor m m notb8 m and m & type > > Almost all iov_iter_rw() callers have the form (iov_iter_rw(iter) == READ) or > (iov_iter_rw(iter) == WRITE). Out of 50-odd callers there are 5 nominal > exceptions: > fs/cifs/smbdirect.c:1936: iov_iter_rw(&msg->msg_iter)); > fs/exfat/inode.c:442: int rw = iov_iter_rw(iter); > fs/f2fs/data.c:3639: int rw = iov_iter_rw(iter); > fs/f2fs/f2fs.h:4082: int rw = iov_iter_rw(iter); > fs/f2fs/f2fs.h:4092: int rw = iov_iter_rw(iter); > > The first one is debugging printk > if (iov_iter_rw(&msg->msg_iter) == WRITE) { > /* It's a bug in upper layer to get there */ > cifs_dbg(VFS, "Invalid msg iter dir %u\n", > iov_iter_rw(&msg->msg_iter)); > rc = -EINVAL; > goto out; > } > and if you look at the condition, the quality of message is > underwhelming - "Data source msg iter passed by caller" would > be more informative. > > Other 4... exfat one is > if (rw == WRITE) { > ... > } > ... > if (ret < 0 && (rw & WRITE)) > exfat_write_failed(mapping, size); > IOW, doing > bool is_write = iov_iter_rw(iter) == WRITE; > would be cleaner. f2fs.h ones are > int rw = iov_iter_rw(iter); > .... > if (.... && rw == WRITE ... > so they are of the same sort (assuming we want that local > variable in the first place). > > f2fs/data.c is the least trivial - it includes things like > if (!down_read_trylock(&fi->i_gc_rwsem[rw])) { > and considering the amount of other stuff done there, > I would suggest something like > int rw = is_data_source(iter) ? WRITE : READ; > > I'll dig myself from under ->d_revalidate() code review, look > through the iov_iter-related series and post review, hopefully > by tonight. Great, thanks Al. Without it being optimised right my other patches keep worsening iov_iter, and I obviously want to avoid that. -- Pavel Begunkov