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Fri, 15 Sep 2023 14:08:40 +0100 From: David Laight To: 'David Howells' CC: Al Viro , Linus Torvalds , Jens Axboe , "Christoph Hellwig" , Christian Brauner , "Matthew Wilcox" , Brendan Higgins , David Gow , "linux-fsdevel@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "kunit-dev@googlegroups.com" , "linux-kernel@vger.kernel.org" , Andrew Morton , Christian Brauner , "David Hildenbrand" , John Hubbard Subject: RE: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC Thread-Topic: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC Thread-Index: AQHZ51kQ26Nrcgq1TkqWWiI5nPv6dLAbdkPggAAj+YCAABqPYP//+bcAgAAdLuD///c+AIAAFogg Date: Fri, 15 Sep 2023 13:08:40 +0000 Message-ID: References: <72e93605b28742c2a496ce4890ecaa80@AcuMS.aculab.com> <5017b9fa177f4deaa5d481a5d8914ab4@AcuMS.aculab.com> <20230914221526.3153402-1-dhowells@redhat.com> <20230914221526.3153402-10-dhowells@redhat.com> <3370515.1694772627@warthog.procyon.org.uk> <3449352.1694776980@warthog.procyon.org.uk> <3585404.1694781366@warthog.procyon.org.uk> In-Reply-To: <3585404.1694781366@warthog.procyon.org.uk> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 11207C0002 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: wd1pj3pqowr8zrzqg6nbbh1yaiyh11qz X-HE-Tag: 1694783343-195879 X-HE-Meta: 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 vu95vzHO sduE7znMoJA7eDg++newFNF1Wx7foNajZLMmLxceUOfEh+Rj8pCBUogggC8TMtCiy0otbmrZr0Fm4BVQJvpHyNGuukqmaWF8tRwcz38b62aRo+goLAlSTmbTXKclObM0eBKAPqZtKBIIlhiO+6znrE2oCxT8XFkIyXk9GIqH+FasQO3BFMMREhjNUCQjXdgu5lY2KkPHTFRZkvG9ZcgToKdnl6hnWyTNkaD9bWxKB/QC1SYnsxznxtnWXTNCjAtIX+luP+fj7KLKWEi3kr7JIH4inZDxxDvivSiIp4prbSYJQYyJB5AlMkZIuKKCiD6TrRcZkoxIWlcJXQozH/1lh4NEeGA== 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: From: David Howells > Sent: 15 September 2023 13:36 >=20 > David Laight wrote: >=20 > > I was thinking of import_iovec() - or whatever its current > > name is. >=20 > That doesn't actually access the buffer described by the iovec[]. >=20 > > That really needs a single structure that contains the iov_iter > > and the cache[] (which the caller pretty much always allocates > > in the same place). >=20 > cache[]? Ah it is usually called iovstack[]. That is the code that reads the iovec[] from user. For small counts there is an on-stack cache[], for large counts it has call kmalloc(). So when the io completes you have to free the allocated buffer. A canonical example is: static ssize_t vfs_readv(struct file *file, const struct iovec __user *vec, =09=09 unsigned long vlen, loff_t *pos, rwf_t flags) { =09struct iovec iovstack[UIO_FASTIOV]; =09struct iovec *iov =3D iovstack; =09struct iov_iter iter; =09ssize_t ret; =09ret =3D import_iovec(ITER_DEST, vec, vlen, ARRAY_SIZE(iovstack), &iov, &= iter); =09if (ret >=3D 0) { =09=09ret =3D do_iter_read(file, &iter, pos, flags); =09=09kfree(iov); =09} =09return ret; } If 'iter' and 'iovstack' are put together in a structure the calling sequence becomes much less annoying. The kfree() can (probably) check iter.iovec !=3D iovsatack (as an inline). But io_uring manages to allocate the iov_iter and iovstack[] in entirely different places - and then copies them about. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)