Re: [PATCH] lib/lz4: add software-pipelined prefetch to decode fast loop

[Arnd Bergmann]

On Tue, Oct 13, 2026, at 05:32, Andrew Morton wrote:
> On Tue, 13 Oct 2026 03:20:10 +0000 Biplab Raut <Biplab.Raut@xxxxxxx> wrote:
>> 
>>   Kernel LZ4 median scan:  1.0994 s
>>   Patched  median scan:    1.0472 s
>>   Median paired speedup:   5.09%  (range 4.19%–6.99%)
>>   Patched was faster in all 15 rounds.
>
> Well that's nice.  I can't actually recall a case where prefetch made a
> measurable difference, although I'm sure there are such cases.
>
> We used to use prefetches often (10+ years ago) but it seems to fallen
> out of favor.  This code is a prime candidate.  mm/ barely uses
> prefetch.  Maybe it should,

Unfortunately the patch causes a build regression on x86 when using lz4
compressed bzImage files:
x86_64-linux-ld: error: unplaced orphan section `.altinstructions' from `arch/x86/boot/compressed/misc.o'
x86_64-linux-ld: error: unplaced orphan section `.altinstr_replacement' from `arch/x86/boot/compressed/misc.o'

I see that the zero-day bot has also reported this[1].

The problem is that the prefetch() helpers use SMP alternatives
that are not available in the decompressor. This has been a problem
in other patches before [2].

Not sure if there is an easy way to stop the decompressor from using
the alternatives here, for 7.4 I assume the only option is to drop
the patch and try again once that is resolved.

       Arnd 

[1] https://lore.kernel.org/all/202610140857.6jRIqXf1-lkp@xxxxxxxxx/
[2] https://lore.kernel.org/all/CAHk-=whrc-ruKs4Kt90EGzKd+pYhZFKs6bgBVCV=55BK+p1nzg@xxxxxxxxxxxxxx/