Re: [PATCH 3/5] mm: add static huge zero folio

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

On Tue, Aug 05, 2025 at 09:33:10AM -0700, Dave Hansen wrote:
> On 8/4/25 05:13, Pankaj Raghav (Samsung) wrote:
> > From: Pankaj Raghav <p.raghav@samsung.com>
> > 
> > There are many places in the kernel where we need to zeroout larger
> > chunks but the maximum segment we can zeroout at a time by ZERO_PAGE
> > is limited by PAGE_SIZE.
> ...
> 
> In x86-land, the rules are pretty clear about using imperative voice.
> There are quite a few "we's" in the changelog and comments in this series.
> 
> I do think they're generally good to avoid and do lead to more clarity,
> but I'm also not sure how important that is in mm-land these days.

Yeah, I will change it to imperative to stay consistent.

<snip>
> >  static inline int split_folio_to_list_to_order(struct folio *folio,
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index e443fe8cd6cf..366a6d2d771e 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -823,6 +823,27 @@ config ARCH_WANT_GENERAL_HUGETLB
> >  config ARCH_WANTS_THP_SWAP
> >  	def_bool n
> >  
> > +config ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO
> > +	def_bool n
> > +
> > +config STATIC_HUGE_ZERO_FOLIO
> > +	bool "Allocate a PMD sized folio for zeroing"
> > +	depends on ARCH_WANTS_STATIC_HUGE_ZERO_FOLIO && TRANSPARENT_HUGEPAGE
> > +	help
> > +	  Without this config enabled, the huge zero folio is allocated on
> > +	  demand and freed under memory pressure once no longer in use.
> > +	  To detect remaining users reliably, references to the huge zero folio
> > +	  must be tracked precisely, so it is commonly only available for mapping
> > +	  it into user page tables.
> > +
> > +	  With this config enabled, the huge zero folio can also be used
> > +	  for other purposes that do not implement precise reference counting:
> > +	  it is still allocated on demand, but never freed, allowing for more
> > +	  wide-spread use, for example, when performing I/O similar to the
> > +	  traditional shared zeropage.
> > +
> > +	  Not suitable for memory constrained systems.
> 
> IMNHO, this is written like a changelog, not documentation for end users
> trying to make sense of Kconfig options. I'd suggest keeping it short
> and sweet:
> 
> config PERSISTENT_HUGE_ZERO_FOLIO
> 	bool "Allocate a persistent PMD-sized folio for zeroing"
> 	...
> 	help
> 	  Enable this option to reduce the runtime refcounting overhead
> 	  of the huge zero folio and expand the places in the kernel
> 	  that can use huge zero folios.
> 
> 	  With this option enabled, the huge zero folio is allocated
> 	  once and never freed. It potentially wastes one huge page
> 	  worth of memory.
> 
> 	  Say Y if your system has lots of memory. Say N if you are
> 	  memory constrained.
> 
This looks short and to the point. I can fold this in the next version.
Thanks.

> >  config MM_ID
> >  	def_bool n
> >  
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index ff06dee213eb..e117b280b38d 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -75,6 +75,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >  static bool split_underused_thp = true;
> >  
> >  static atomic_t huge_zero_refcount;
> > +atomic_t huge_zero_folio_is_static __read_mostly;
> >  struct folio *huge_zero_folio __read_mostly;
> >  unsigned long huge_zero_pfn __read_mostly = ~0UL;
> >  unsigned long huge_anon_orders_always __read_mostly;
> > @@ -266,6 +267,45 @@ void mm_put_huge_zero_folio(struct mm_struct *mm)
> >  		put_huge_zero_folio();
> >  }
> >  
> > +#ifdef CONFIG_STATIC_HUGE_ZERO_FOLIO
> > +
> > +struct folio *__get_static_huge_zero_folio(void)
> > +{
> > +	static unsigned long fail_count_clear_timer;
> > +	static atomic_t huge_zero_static_fail_count __read_mostly;
> > +
> > +	if (unlikely(!slab_is_available()))
> > +		return NULL;
> > +
> > +	/*
> > +	 * If we failed to allocate a huge zero folio, just refrain from
> > +	 * trying for one minute before retrying to get a reference again.
> > +	 */
> > +	if (atomic_read(&huge_zero_static_fail_count) > 1) {
> > +		if (time_before(jiffies, fail_count_clear_timer))
> > +			return NULL;
> > +		atomic_set(&huge_zero_static_fail_count, 0);
> > +	}
> 
> Any reason that this is an open-coded ratelimit instead of using
> 'struct ratelimit_state'?
> 
> I also find the 'huge_zero_static_fail_count' use pretty unintuitive.
> This is fundamentally a slow path. Ideally, it's called once. In the
> pathological case, it's called once a minute.
> 
> I'd probably just recommend putting a rate limit on this function, then
> using a plain old mutex for the actual allocation to keep multiple
> threads out.
> 
> Then the function becomes something like this:
> 
> 	if (__ratelimit(&huge_zero_alloc_ratelimit))
> 		return;
> 
> 	guard(mutex)(&huge_zero_mutex);
> 
> 	if (!get_huge_zero_folio())
> 		return NULL;
> 
> 	static_key_enable(&huge_zero_noref_key);
> 
> 	return huge_zero_folio;
> 
> No atomic, no cmpxchg, no races on allocating.

David already reworked this part based on Lorenzo's feedback (he also
did not like the ratelimiting part like you). The reworked diff is
here[1]. No ratelimiting, etc.

> 
> 
> ...
> >  static unsigned long shrink_huge_zero_folio_count(struct shrinker *shrink,
> >  						  struct shrink_control *sc)
> >  {
> > @@ -277,7 +317,11 @@ static unsigned long shrink_huge_zero_folio_scan(struct shrinker *shrink,
> >  						 struct shrink_control *sc)
> >  {
> >  	if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
> > -		struct folio *zero_folio = xchg(&huge_zero_folio, NULL);
> > +		struct folio *zero_folio;
> > +
> > +		if (WARN_ON_ONCE(atomic_read(&huge_zero_folio_is_static)))
> > +			return 0;
> > +		zero_folio = xchg(&huge_zero_folio, NULL);
> >  		BUG_ON(zero_folio == NULL);
> >  		WRITE_ONCE(huge_zero_pfn, ~0UL);
> >  		folio_put(zero_folio);

> 
> This seems like a hack to me. If you don't want the shrinker to run,
> then deregister it. Keeping the refcount elevated is fine, but
> repeatedly calling the shrinker to do atomic_cmpxchg() when you *know*
> it will do nothing seems silly.
> 

The new version[1] deregisters instead of having this condition. :)

> If you can't deregister the shrinker, at least use the static_key
> approach and check the static key instead of doing futile cmpxchg's forever.

--
Pankaj

[1] https://lore.kernel.org/linux-mm/70049abc-bf79-4d04-a0a8-dd3787195986@redhat.com/