Le 03/09/2020 à 09:11, Christoph Hellwig a écrit :

On Wed, Sep 02, 2020 at 11:02:22AM -0700, Linus Torvalds wrote:

quoted

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.

quoted

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).

quoted

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