Subject: Re: Large modules with 6.15 [was: [PATCH v4 6/6] percpu/x86: Enable strict percpu checks via named AS qualifiers]

From: "Dr. David Alan Gilbert" <[email-masked]>
Date: Thu, 5 Jun 2025 14:39:41 +0000

* Jiri Slaby ([email-masked]) wrote:
> On 27. 01. 25, 17:05, Uros Bizjak wrote:
> > This patch declares percpu variables in __seg_gs/__seg_fs named AS
> > and keeps them named AS qualified until they are dereferenced with
> > percpu accessor. This approach enables various compiler check
> > for cross-namespace variable assignments.
> > 
> > Signed-off-by: Uros Bizjak <[email-masked]>
> > Acked-by: Nadav Amit <[email-masked]>
> > Cc: Dennis Zhou <[email-masked]>
> > Cc: Tejun Heo <[email-masked]>
> > Cc: Christoph Lameter <[email-masked]>
> > Cc: Thomas Gleixner <[email-masked]>
> > Cc: Ingo Molnar <[email-masked]>
> > Cc: Borislav Petkov <[email-masked]>
> > Cc: Dave Hansen <[email-masked]>
> > Cc: "H. Peter Anvin" <[email-masked]>
> > Cc: Linus Torvalds <[email-masked]>
> > Cc: Andy Lutomirski <[email-masked]>
> > Cc: Brian Gerst <[email-masked]>
> > Cc: Peter Zijlstra <[email-masked]>
> > ---
> >   arch/x86/include/asm/percpu.h | 15 ++++++++++++---
> >   1 file changed, 12 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> > index 27f668660abe..474d648bca9a 100644
> > --- a/arch/x86/include/asm/percpu.h
> > +++ b/arch/x86/include/asm/percpu.h
> > @@ -95,9 +95,18 @@
> >   #endif /* CONFIG_SMP */
> > -#define __my_cpu_type(var)	typeof(var) __percpu_seg_override
> > -#define __my_cpu_ptr(ptr)	(__my_cpu_type(*(ptr))*)(__force uintptr_t)(ptr)
> > -#define __my_cpu_var(var)	(*__my_cpu_ptr(&(var)))
> > +#if defined(CONFIG_USE_X86_SEG_SUPPORT) && defined(USE_TYPEOF_UNQUAL)
> > +# define __my_cpu_type(var)	typeof(var)
> > +# define __my_cpu_ptr(ptr)	(ptr)
> > +# define __my_cpu_var(var)	(var)
> > +
> > +# define __percpu_qual		__percpu_seg_override
> > +#else
> > +# define __my_cpu_type(var)	typeof(var) __percpu_seg_override
> > +# define __my_cpu_ptr(ptr)	(__my_cpu_type(*(ptr))*)(__force uintptr_t)(ptr)
> > +# define __my_cpu_var(var)	(*__my_cpu_ptr(&(var)))
> > +#endif
> > +
> 
> Another issue with this is this causes all modules in 6.15 are 2-4 times
> (compressed size) bigger:

Oh, is that why config-all kernel is too big?

Dave

> $ ll /usr/lib/modules/*-[0-9]-default/kernel/drivers/atm/atmtcp.ko.zst
> > -rw-r--r--. 1 root root 10325 May 13 11:49
> /usr/lib/modules/6.14.6-2-default/kernel/drivers/atm/atmtcp.ko.zst
> > -rw-r--r--. 1 root root 39677 Jun  2 09:13
> /usr/lib/modules/6.15.0-1-default/kernel/drivers/atm/atmtcp.ko.zst
> 
> It's due to larger .BTF section:
> .BTF              PROGBITS         0000000000000000  [-00003080-]
> [-       00000000000011a8-]  {+00003100+}
> {+       0000000000012cf8+}  0000000000000000           0     0     1
> 
> There are a lot of new BTF types defined in each module like:
> +attribute_group STRUCT
> +backing_dev_info STRUCT
> +bdi_writeback STRUCT
> +bin_attribute STRUCT
> +bio_end_io_t TYPEDEF
> +bio_list STRUCT
> +bio_set STRUCT
> +bio STRUCT
> +bio_vec STRUCT
> 
> Reverting this gives me back to normal sizes.
> 
> Any ideas?
> 
> FTR downstream report:
> https://bugzilla.suse.com/show_bug.cgi?id=1244135
> 
> thanks,
> -- 
> js
> suse labs
> 
> 
-- 
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