From: David Laight <David.Laight@ACULAB.COM>
To: 'Al Viro' <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>,
Jens Axboe <axboe@kernel.dk>, Arnd Bergmann <arnd@arndb.de>,
David Howells <dhowells@redhat.com>,
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<linux-arm-kernel@lists.infradead.org>,
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Subject: RE: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw()
Date: Mon, 21 Sep 2020 15:44:00 +0000 [thread overview]
Message-ID: <226e03bf941844eba4d64af31633c177@AcuMS.aculab.com> (raw)
In-Reply-To: <20200921152937.GX3421308@ZenIV.linux.org.uk>
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)
next prev parent reply other threads:[~2020-09-21 15:44 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-21 14:34 let import_iovec deal with compat_iovecs as well v2 Christoph Hellwig
2020-09-21 14:34 ` [PATCH 01/11] compat.h: fix a spelling error in <linux/compat.h> Christoph Hellwig
2020-09-21 14:34 ` [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw() Christoph Hellwig
2020-09-21 14:48 ` Matthew Wilcox
2020-09-21 15:02 ` Al Viro
2020-09-21 15:21 ` David Laight
2020-09-21 15:29 ` Al Viro
2020-09-21 15:44 ` David Laight [this message]
2020-09-21 16:27 ` Al Viro
2020-09-21 16:12 ` Christoph Hellwig
2020-09-21 14:34 ` [PATCH 03/11] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c and mark it static Christoph Hellwig
2020-09-21 14:34 ` [PATCH 04/11] iov_iter: explicitly check for CHECK_IOVEC_ONLY in rw_copy_check_uvector Christoph Hellwig
2020-09-21 15:05 ` David Laight
2020-09-21 15:11 ` Al Viro
2020-09-21 15:26 ` David Laight
2020-09-21 15:07 ` Al Viro
2020-09-21 14:34 ` [PATCH 05/11] iov_iter: merge the compat case into rw_copy_check_uvector Christoph Hellwig
2020-09-21 15:14 ` Al Viro
2021-01-08 11:49 ` David Laight
2020-09-21 14:34 ` [PATCH 06/11] iov_iter: handle the compat case in import_iovec Christoph Hellwig
2020-09-21 15:20 ` Al Viro
2020-09-21 14:34 ` [PATCH 07/11] fs: remove various compat readv/writev helpers Christoph Hellwig
2020-09-21 14:34 ` [PATCH 08/11] fs: remove the compat readv/writev syscalls Christoph Hellwig
2020-09-21 14:34 ` [PATCH 09/11] fs: remove compat_sys_vmsplice Christoph Hellwig
2020-09-21 14:34 ` [PATCH 10/11] mm: remove compat_process_vm_{readv,writev} Christoph Hellwig
2020-09-21 14:34 ` [PATCH 11/11] security/keys: remove compat_keyctl_instantiate_key_iov Christoph Hellwig
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