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Wed, 27 Jan 2021 07:51:51 -0800 (PST) Subject: Re: [PATCH] iov_iter: optimise iter type checking To: Al Viro Cc: David Laight , "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> <1783c58f-1016-0c6b-be7f-a93bc2f8f2a4@gmail.com> <20210116051818.GF3579531@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== Message-ID: Date: Wed, 27 Jan 2021 15:48:10 +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: <20210116051818.GF3579531@ZenIV.linux.org.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/01/2021 05:18, Al Viro wrote: > On Sat, Jan 09, 2021 at 10:11:09PM +0000, Pavel Begunkov wrote: > >>> Does any code actually look at the fields as a pair? >>> Would it even be better to use separate bytes? >>> Even growing the on-stack structure by a word won't really matter. >> >> u8 type, rw; >> >> That won't bloat the struct. I like the idea. If used together compilers >> can treat it as u16. > > Reasonable, and from what I remember from looking through the users, > no readers will bother with looking at both at the same time. Al, are you going turn it into a patch, or prefer me to take over? > On the write side... it's only set in iov_iter_{kvec,bvec,pipe,discard,init}. > I sincerely doubt anyone would give a fuck, not to mention that something > like > void iov_iter_pipe(struct iov_iter *i, unsigned int direction, > struct pipe_inode_info *pipe, > size_t count) > { > BUG_ON(direction != READ); > WARN_ON(pipe_full(pipe->head, pipe->tail, pipe->ring_size)); > *i = (struct iov_iter) { > .iter_type = ITER_PIPE, > .data_source = false, > .pipe = pipe, > .head = pipe->head, > .start_head = pipe->head, > .count = count, > .iov_offset = 0 > }; > } > > would make more sense anyway - we do want to overwrite everything in the > object, and let the compiler do whatever it likes to do. > > So... something like (completely untested) variant below, perhaps? > > diff --git a/include/linux/uio.h b/include/linux/uio.h > index 72d88566694e..ed8ad2c5d384 100644 > --- a/include/linux/uio.h > +++ b/include/linux/uio.h > @@ -19,20 +19,16 @@ struct kvec { > > enum iter_type { > /* iter types */ > - ITER_IOVEC = 4, > - ITER_KVEC = 8, > - ITER_BVEC = 16, > - ITER_PIPE = 32, > - ITER_DISCARD = 64, > + ITER_IOVEC, > + ITER_KVEC, > + ITER_BVEC, > + ITER_PIPE, > + ITER_DISCARD > }; > > struct iov_iter { > - /* > - * Bit 0 is the read/write bit, set if we're writing. > - * Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and > - * the caller isn't expecting to drop a page reference when done. > - */ > - unsigned int type; > + u8 iter_type; > + bool data_source; > size_t iov_offset; > size_t count; > union { > @@ -52,7 +48,7 @@ struct iov_iter { > > static inline enum iter_type iov_iter_type(const struct iov_iter *i) > { > - return i->type & ~(READ | WRITE); > + return i->iter_type; > } > > static inline bool iter_is_iovec(const struct iov_iter *i) > @@ -82,7 +78,7 @@ static inline bool iov_iter_is_discard(const struct iov_iter *i) > > static inline unsigned char iov_iter_rw(const struct iov_iter *i) > { > - return i->type & (READ | WRITE); > + return i->data_source ? WRITE : READ; > } > > /* > diff --git a/lib/iov_iter.c b/lib/iov_iter.c > index 1635111c5bd2..133c03b2dcae 100644 > --- a/lib/iov_iter.c > +++ b/lib/iov_iter.c > @@ -81,19 +81,18 @@ > #define iterate_all_kinds(i, n, v, I, B, K) { \ > if (likely(n)) { \ > size_t skip = i->iov_offset; \ > - if (unlikely(i->type & ITER_BVEC)) { \ > + if (likely(i->iter_type == ITER_IOVEC)) { \ > + const struct iovec *iov; \ > + struct iovec v; \ > + iterate_iovec(i, n, v, iov, skip, (I)) \ > + } else if (i->iter_type == ITER_BVEC) { \ > struct bio_vec v; \ > struct bvec_iter __bi; \ > iterate_bvec(i, n, v, __bi, skip, (B)) \ > - } else if (unlikely(i->type & ITER_KVEC)) { \ > + } else if (i->iter_type == ITER_KVEC) { \ > const struct kvec *kvec; \ > struct kvec v; \ > iterate_kvec(i, n, v, kvec, skip, (K)) \ > - } else if (unlikely(i->type & ITER_DISCARD)) { \ > - } else { \ > - const struct iovec *iov; \ > - struct iovec v; \ > - iterate_iovec(i, n, v, iov, skip, (I)) \ > } \ > } \ > } > @@ -103,7 +102,17 @@ > n = i->count; \ > if (i->count) { \ > size_t skip = i->iov_offset; \ > - if (unlikely(i->type & ITER_BVEC)) { \ > + if (likely(i->iter_type == ITER_IOVEC)) { \ > + const struct iovec *iov; \ > + struct iovec v; \ > + iterate_iovec(i, n, v, iov, skip, (I)) \ > + if (skip == iov->iov_len) { \ > + iov++; \ > + skip = 0; \ > + } \ > + i->nr_segs -= iov - i->iov; \ > + i->iov = iov; \ > + } else if (i->iter_type == ITER_BVEC) { \ > const struct bio_vec *bvec = i->bvec; \ > struct bio_vec v; \ > struct bvec_iter __bi; \ > @@ -111,7 +120,7 @@ > i->bvec = __bvec_iter_bvec(i->bvec, __bi); \ > i->nr_segs -= i->bvec - bvec; \ > skip = __bi.bi_bvec_done; \ > - } else if (unlikely(i->type & ITER_KVEC)) { \ > + } else if (i->iter_type == ITER_KVEC) { \ > const struct kvec *kvec; \ > struct kvec v; \ > iterate_kvec(i, n, v, kvec, skip, (K)) \ > @@ -121,18 +130,8 @@ > } \ > i->nr_segs -= kvec - i->kvec; \ > i->kvec = kvec; \ > - } else if (unlikely(i->type & ITER_DISCARD)) { \ > + } else if (i->iter_type == ITER_DISCARD) { \ > skip += n; \ > - } else { \ > - const struct iovec *iov; \ > - struct iovec v; \ > - iterate_iovec(i, n, v, iov, skip, (I)) \ > - if (skip == iov->iov_len) { \ > - iov++; \ > - skip = 0; \ > - } \ > - i->nr_segs -= iov - i->iov; \ > - i->iov = iov; \ > } \ > i->count -= n; \ > i->iov_offset = skip; \ > @@ -434,7 +433,7 @@ int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes) > int err; > struct iovec v; > > - if (!(i->type & (ITER_BVEC|ITER_KVEC))) { > + if (i->iter_type == ITER_IOVEC) { > iterate_iovec(i, bytes, v, iov, skip, ({ > err = fault_in_pages_readable(v.iov_base, v.iov_len); > if (unlikely(err)) > @@ -450,19 +449,26 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction, > size_t count) > { > WARN_ON(direction & ~(READ | WRITE)); > - direction &= READ | WRITE; > > /* It will get better. Eventually... */ > - if (uaccess_kernel()) { > - i->type = ITER_KVEC | direction; > - i->kvec = (struct kvec *)iov; > - } else { > - i->type = ITER_IOVEC | direction; > - i->iov = iov; > - } > - i->nr_segs = nr_segs; > - i->iov_offset = 0; > - i->count = count; > + if (uaccess_kernel()) > + *i = (struct iov_iter) { > + .iter_type = ITER_KVEC, > + .data_source = direction, > + .kvec = (struct kvec *)iov, > + .nr_segs = nr_segs, > + .iov_offset = 0, > + .count = count > + }; > + else > + *i = (struct iov_iter) { > + .iter_type = ITER_IOVEC, > + .data_source = direction, > + .iov = iov, > + .nr_segs = nr_segs, > + .iov_offset = 0, > + .count = count > + }; > } > EXPORT_SYMBOL(iov_iter_init); > > @@ -915,17 +921,20 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes, > { > if (unlikely(!page_copy_sane(page, offset, bytes))) > return 0; > - if (i->type & (ITER_BVEC|ITER_KVEC)) { > + if (likely(i->iter_type == ITER_IOVEC)) > + return copy_page_to_iter_iovec(page, offset, bytes, i); > + if (i->iter_type == ITER_BVEC || i->iter_type == ITER_KVEC) { > void *kaddr = kmap_atomic(page); > size_t wanted = copy_to_iter(kaddr + offset, bytes, i); > kunmap_atomic(kaddr); > return wanted; > - } else if (unlikely(iov_iter_is_discard(i))) > - return bytes; > - else if (likely(!iov_iter_is_pipe(i))) > - return copy_page_to_iter_iovec(page, offset, bytes, i); > - else > + } > + if (i->iter_type == ITER_PIPE) > return copy_page_to_iter_pipe(page, offset, bytes, i); > + if (i->iter_type == ITER_DISCARD) > + return bytes; > + WARN_ON(1); > + return 0; > } > EXPORT_SYMBOL(copy_page_to_iter); > > @@ -934,17 +943,16 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes, > { > if (unlikely(!page_copy_sane(page, offset, bytes))) > return 0; > - if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) { > - WARN_ON(1); > - return 0; > - } > - if (i->type & (ITER_BVEC|ITER_KVEC)) { > + if (likely(i->iter_type == ITER_IOVEC)) > + return copy_page_from_iter_iovec(page, offset, bytes, i); > + if (i->iter_type == ITER_BVEC || i->iter_type == ITER_KVEC) { > void *kaddr = kmap_atomic(page); > size_t wanted = _copy_from_iter(kaddr + offset, bytes, i); > kunmap_atomic(kaddr); > return wanted; > - } else > - return copy_page_from_iter_iovec(page, offset, bytes, i); > + } > + WARN_ON(1); > + return 0; > } > EXPORT_SYMBOL(copy_page_from_iter); > > @@ -1172,11 +1180,14 @@ void iov_iter_kvec(struct iov_iter *i, unsigned int direction, > size_t count) > { > WARN_ON(direction & ~(READ | WRITE)); > - i->type = ITER_KVEC | (direction & (READ | WRITE)); > - i->kvec = kvec; > - i->nr_segs = nr_segs; > - i->iov_offset = 0; > - i->count = count; > + *i = (struct iov_iter) { > + .iter_type = ITER_KVEC, > + .data_source = direction, > + .kvec = kvec, > + .nr_segs = nr_segs, > + .iov_offset = 0, > + .count = count > + }; > } > EXPORT_SYMBOL(iov_iter_kvec); > > @@ -1185,11 +1196,14 @@ void iov_iter_bvec(struct iov_iter *i, unsigned int direction, > size_t count) > { > WARN_ON(direction & ~(READ | WRITE)); > - i->type = ITER_BVEC | (direction & (READ | WRITE)); > - i->bvec = bvec; > - i->nr_segs = nr_segs; > - i->iov_offset = 0; > - i->count = count; > + *i = (struct iov_iter) { > + .iter_type = ITER_BVEC, > + .data_source = direction, > + .bvec = bvec, > + .nr_segs = nr_segs, > + .iov_offset = 0, > + .count = count > + }; > } > EXPORT_SYMBOL(iov_iter_bvec); > > @@ -1199,12 +1213,15 @@ void iov_iter_pipe(struct iov_iter *i, unsigned int direction, > { > BUG_ON(direction != READ); > WARN_ON(pipe_full(pipe->head, pipe->tail, pipe->ring_size)); > - i->type = ITER_PIPE | READ; > - i->pipe = pipe; > - i->head = pipe->head; > - i->iov_offset = 0; > - i->count = count; > - i->start_head = i->head; > + *i = (struct iov_iter) { > + .iter_type = ITER_PIPE, > + .data_source = false, > + .pipe = pipe, > + .head = pipe->head, > + .start_head = pipe->head, > + .count = count, > + .iov_offset = 0 > + }; > } > EXPORT_SYMBOL(iov_iter_pipe); > > @@ -1220,9 +1237,11 @@ EXPORT_SYMBOL(iov_iter_pipe); > void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count) > { > BUG_ON(direction != READ); > - i->type = ITER_DISCARD | READ; > - i->count = count; > - i->iov_offset = 0; > + *i = (struct iov_iter) { > + .iter_type = ITER_DISCARD, > + .data_source = false, > + .count = count, > + }; > } > EXPORT_SYMBOL(iov_iter_discard); > > -- Pavel Begunkov