From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Laight Date: Mon, 21 Sep 2020 15:44:00 +0000 Subject: RE: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw() Message-Id: <226e03bf941844eba4d64af31633c177@AcuMS.aculab.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit List-Id: References: <20200921143434.707844-1-hch@lst.de> <20200921143434.707844-3-hch@lst.de> <20200921150211.GS3421308@ZenIV.linux.org.uk> <20200921152937.GX3421308@ZenIV.linux.org.uk> In-Reply-To: <20200921152937.GX3421308@ZenIV.linux.org.uk> To: 'Al Viro' Cc: Christoph Hellwig , Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" From: Al Viro > Sent: 21 September 2020 16:30 > > On Mon, Sep 21, 2020 at 03:21:35PM +0000, David Laight wrote: > > > You really don't want to be looping through the array twice. > > Profiles, please. I did some profiling of send() v sendmsg() much earlier in the year. I can't remember the exact details but the extra cost of sendmsg() is far more than you might expect. (I was timing sending fully built IPv4 UDP packets using a raw socket.) About half the difference does away if you change the copy_from_user() to __copy_from_user() when reading the struct msghdr and iov[] from userspace (user copy hardening is expensive). The rest is just code path, my gut feeling is that a lot of that is in import_iovec(). Remember semdmsg() is likely to be called with an iov count of 1. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)