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From: Al Viro <viro@zeniv.linux.org.uk>
To: David Laight <David.Laight@aculab.com>
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>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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	"linux-security-module@vger.kernel.org" 
	<linux-security-module@vger.kernel.org>
Subject: Re: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw()
Date: Mon, 21 Sep 2020 17:27:53 +0100	[thread overview]
Message-ID: <20200921162753.GY3421308@ZenIV.linux.org.uk> (raw)
In-Reply-To: <226e03bf941844eba4d64af31633c177@AcuMS.aculab.com>

On Mon, Sep 21, 2020 at 03:44:00PM +0000, David Laight wrote:
> 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).

Wha...?  So the difference is within 4 times the overhead of the
hardening checks done for one call of copy_from_user()?

> 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.

... which makes that loop unlikely to be noticable in the entire
mess, whether you pass it once or twice.  IOW, unless you can show
profiles where that loop is sufficiently hot or if you can show
the timings change from splitting it in two, I'll remain very
sceptical about that assertion.

  reply	other threads:[~2020-09-21 16:28 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
2020-09-21 16:27           ` Al Viro [this message]
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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