Re: [PATCH] mm: Fix clang W=1 compiler warnings

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Feb 07, 2025 at 05:38:13PM -0800, Jakub Kicinski wrote:
> On Fri, 7 Feb 2025 17:01:00 -0800 Linus Torvalds wrote:
> > On Fri, 7 Feb 2025 at 16:49, Jakub Kicinski <kuba@kernel.org> wrote:
> > > Could we possibly please still consider taking this in for 6.14? :(
> > > Since the warning comes from vmstat.h pretty much every object file
> > > generates this warning. clang 19 is getting more widely used now,
> > > its making it hard to see new warnings.  
> > 
> > So:
> > 
> >  - I build the kernel with clang, but I don't have clang-19, so it's
> > kind of pointless sending patches that DO NOT EVEN EXPLAIN WHAT THE
> > WARNINGS ARE.
> > 
> >  - and even if you explain *WHAT* the warnings are, please also
> > explain *WHY* they are valid and should be cared about.
> > 
> > Because honestly, W=1 is literally meant for "warnings that aren't
> > necessarily valid". That's why they aren't on by default.
> > 
> > So no, I'm certainly not applying unexplained random patches that
> > don't bother to explain the what or the why. Not for 6.14, not ever.
> 
> I thought I'd ask.. :) FWIW this for every single object:
> 
>   CC      net/core/request_sock.o
> In file included from ../net/core/request_sock.c:14:
> In file included from ../include/linux/tcp.h:17:
> In file included from ../include/linux/skbuff.h:17:
> In file included from ../include/linux/bvec.h:10:
> In file included from ../include/linux/highmem.h:8:
> In file included from ../include/linux/cacheflush.h:5:
> In file included from ../arch/x86/include/asm/cacheflush.h:5:
> In file included from ../include/linux/mm.h:2224:
> ../include/linux/vmstat.h:504:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
>   504 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
>       |                            ~~~~~~~~~~~~~~~~~~~~~ ^
>   505 |                            item];
>       |                            ~~~~
> ../include/linux/vmstat.h:511:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
>   511 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
>       |                            ~~~~~~~~~~~~~~~~~~~~~ ^
>   512 |                            NR_VM_NUMA_EVENT_ITEMS +
>       |                            ~~~~~~~~~~~~~~~~~~~~~~
> ../include/linux/vmstat.h:524:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
>   524 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
>       |                            ~~~~~~~~~~~~~~~~~~~~~ ^
>   525 |                            NR_VM_NUMA_EVENT_ITEMS +
>       |                            ~~~~~~~~~~~~~~~~~~~~~~
> 3 warnings generated.
> 
> > Fix the patch. Explain the problem. And possibly just disable the warning.
> 
> That'd be great. Nathan, would that be okay with you?

FWIW, I sent a patch to move this warning to W=2 in October and pinged
it in December:

https://lore.kernel.org/20241017-disable-two-clang-enum-warnings-v2-1-163ac04346ae@kernel.org/

'b4 shazam' tells me that it is still applicable on Linus's tree so
maybe he can just apply it directly?

Cheers,
Nathan