Re: [PATCH] mm/page_alloc: Work around a pahole limitation with zero-sized struct pagesets

[Mel Gorman]

On Thu, May 27, 2021 at 09:04:24AM +0100, Christoph Hellwig wrote:
> On Wed, May 26, 2021 at 09:07:41AM +0100, Mel Gorman wrote:
> > +    !defined(CONFIG_DEBUG_LOCK_ALLOC) &&		\
> > +    !defined(CONFIG_PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT)
> > +	/*
> > +	 * pahole 1.21 and earlier gets confused by zero-sized per-CPU
> > +	 * variables and produces invalid BTF. Ensure that
> > +	 * sizeof(struct pagesets) != 0 for older versions of pahole.
> > +	 */
> > +	char __pahole_hack;
> > +	#warning "pahole too old to support zero-sized struct pagesets"
> > +#endif
> 
> Err, hell no.  We should not mess up the kernel for broken tools that
> are not relevant to the kernel build itself ever.

What do you suggest as an alternative?

I added Arnaldo to the cc as he tagged the last released version of
pahole (1.21) and may be able to tag a 1.22 with Andrii's fix for pahole
included.

The most obvious alternative fix for this issue is to require pahole
1.22 to set CONFIG_DEBUG_INFO_BTF but obviously a version 1.22 that works
needs to exist first and right now it does not. I'd be ok with this but
users of DEBUG_INFO_BTF may object given that it'll be impossible to set
the option until there is a release.

The second alternative fix is to embed the local_lock
within struct per_cpu_pages. It was shown this was possible in
https://lore.kernel.org/linux-rt-users/20210419141341.26047-1-mgorman@xxxxxxxxxxxxxxxxxxx/T/#md1001d7af52ac0d6d214b95e98fe051f9399de64
but I dropped it because it makes the locking protocol complex e.g.
config-specific lock-switchin in free_unref_page_list.

The last one is wrapping local_lock behind #defines and only defining the
per-cpu structures when local_lock_t is a non-zero size. That is simply
too ugly for words, the locking patterns should always be the same.


-- 
Mel Gorman
SUSE Labs