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

[Christophe Leroy]

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