Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Christoph Hellwig <hch@lst.de>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Michael Ellerman <mpe@ellerman.id.au>,
	the arch/x86 maintainers <x86@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()
Date: Thu, 3 Sep 2020 09:27:04 +0200	[thread overview]
Message-ID: <e6afe576-c3b2-81af-b042-e5930a8fd4c8@csgroup.eu> (raw)
In-Reply-To: <20200903071144.GA19247@lst.de>

Le 03/09/2020 à 09:11, Christoph Hellwig a écrit :
> On Wed, Sep 02, 2020 at 11:02:22AM -0700, Linus Torvalds wrote:
>> I don't see why this change would make any difference.
> 
> Me neither, but while looking at a different project I did spot places
> that actually do an access_ok with len 0, that's why I wanted him to
> try.
> 
> That being said: Christophe are these number stables?  Do you get
> similar numbers with multiple runs?

Yes the numbers are similar with multiple runs and multiple reboots.

> 
>> And btw, why do the 32-bit and 64-bit checks even differ? It's not
>> like the extra (single) instruction should even matter. I think the
>> main reason is that the simpler 64-bit case could stay as a macro
>> (because it only uses "addr" and "size" once), but honestly, that
>> "simplification" doesn't help when you then need to have that #ifdef
>> for the 32-bit case and an inline function anyway.
> 
> I'll have to leave that to the powerpc folks.  The intent was to not
> change the behavior (and I even fucked that up for the the size == 0
> case).
> 
>> However, I suspect a bigger reason for the actual performance
>> degradation would be the patch that makes things use "write_iter()"
>> for writing, even when a simpler "write()" exists.
> 
> Except that we do not actually have such a patch.  For normal user
> writes we only use ->write_iter if ->write is not present.  But what
> shows up in the profile is that /dev/zero only has a read_iter op and
> not a normal read.  I've added a patch below that implements a normal
> read which might help a tad with this workload, but should not be part
> of a regression.
> 
> Also Christophe:  can you bisect which patch starts this?  Is it really
> this last patch in the series?

5.9-rc2: 91.5MB/s
Patch 1: 74.9MB/s
Patch 2: 97.9MB/s
Patch 3: 97.7MB/s
Patch 4 to 9: 97.9MB/s
Patch 10: 85.3MB/s
Patch 11: 75.4MB/s

See my other mail, when removing CONFIG_STACKPROTECTOR, I get a stable 
99.8MB/s throughput.

Christophe