Re: [PATCH v2 3/5] mm: add persistent huge zero folio

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Aug 08, 2025 at 02:11:39PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
>
> Many places in the kernel need to zero out larger chunks, but the
> maximum segment that can be zeroed out at a time by ZERO_PAGE is limited
> by PAGE_SIZE.
>
> This is especially annoying in block devices and filesystems where
> multiple ZERO_PAGEs are attached to the bio in different bvecs. With
> multipage bvec support in block layer, it is much more efficient to send
> out larger zero pages as a part of single bvec.
>
> This concern was raised during the review of adding Large Block Size
> support to XFS[1][2].
>
> Usually huge_zero_folio is allocated on demand, and it will be
> deallocated by the shrinker if there are no users of it left. At moment,
> huge_zero_folio infrastructure refcount is tied to the process lifetime
> that created it. This might not work for bio layer as the completions
> can be async and the process that created the huge_zero_folio might no
> longer be alive. And, one of the main points that came up during
> discussion is to have something bigger than zero page as a drop-in
> replacement.
>
> Add a config option PERSISTENT_HUGE_ZERO_FOLIO that will result in
> allocating the huge zero folio during early init and never free the memory
> by disabling the shrinker. This makes using the huge_zero_folio without
> having to pass any mm struct and does not tie the lifetime of the zero
> folio to anything, making it a drop-in replacement for ZERO_PAGE.
>
> If PERSISTENT_HUGE_ZERO_FOLIO config option is enabled, then
> mm_get_huge_zero_folio() will simply return the allocated page instead of
> dynamically allocating a new PMD page.
>
> Use this option carefully in resource constrained systems as it uses
> one full PMD sized page for zeroing purposes.
>
> [1] https://lore.kernel.org/linux-xfs/20231027051847.GA7885@lst.de/
> [2] https://lore.kernel.org/linux-xfs/ZitIK5OnR7ZNY0IG@infradead.org/
>
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>

This is much nicer and now _super_ simple, I like it.

A few nits below but generally:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  include/linux/huge_mm.h | 16 ++++++++++++++++
>  mm/Kconfig              | 16 ++++++++++++++++
>  mm/huge_memory.c        | 40 ++++++++++++++++++++++++++++++----------
>  3 files changed, 62 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..bd547857c6c1 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -495,6 +495,17 @@ static inline bool is_huge_zero_pmd(pmd_t pmd)
>  struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
>  void mm_put_huge_zero_folio(struct mm_struct *mm);
>
> +static inline struct folio *get_persistent_huge_zero_folio(void)
> +{
> +	if (!IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO))
> +		return NULL;
> +
> +	if (unlikely(!huge_zero_folio))
> +		return NULL;
> +
> +	return huge_zero_folio;
> +}
> +
>  static inline bool thp_migration_supported(void)
>  {
>  	return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
> @@ -685,6 +696,11 @@ static inline int change_huge_pud(struct mmu_gather *tlb,
>  {
>  	return 0;
>  }
> +
> +static inline struct folio *get_persistent_huge_zero_folio(void)
> +{
> +	return NULL;
> +}
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>
>  static inline int split_folio_to_list_to_order(struct folio *folio,
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e443fe8cd6cf..fbe86ef97fd0 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -823,6 +823,22 @@ config ARCH_WANT_GENERAL_HUGETLB
>  config ARCH_WANTS_THP_SWAP
>  	def_bool n
>
> +config PERSISTENT_HUGE_ZERO_FOLIO
> +	bool "Allocate a PMD sized folio for zeroing"
> +	depends on TRANSPARENT_HUGEPAGE

I feel like we really need to sort out what is/isn't predicated on THP... it
seems like THP is sort of short hand for 'any large folio stuff' but not
always...

But this is a more general point :)

> +	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. This can potentially improve
> +	  the performance while performing an I/O.

NIT: I think we can drop 'an', and probably refactor this sentence to something
like 'For instance, block I/O benefits from access to large folios for zeroing
memory'.

> +
> +	  With this option enabled, the huge zero folio is allocated
> +	  once and never freed. One full huge page worth of memory shall
> +	  be used.

NIT: huge page worth -> huge page's worth

> +
> +	  Say Y if your system has lots of memory. Say N if you are
> +	  memory constrained.
> +
>  config MM_ID
>  	def_bool n
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ff06dee213eb..bedda9640936 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -248,6 +248,9 @@ static void put_huge_zero_folio(void)
>
>  struct folio *mm_get_huge_zero_folio(struct mm_struct *mm)
>  {
> +	if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO))
> +		return huge_zero_folio;
> +
>  	if (test_bit(MMF_HUGE_ZERO_FOLIO, &mm->flags))
>  		return READ_ONCE(huge_zero_folio);
>
> @@ -262,6 +265,9 @@ struct folio *mm_get_huge_zero_folio(struct mm_struct *mm)
>
>  void mm_put_huge_zero_folio(struct mm_struct *mm)
>  {
> +	if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO))
> +		return;
> +
>  	if (test_bit(MMF_HUGE_ZERO_FOLIO, &mm->flags))
>  		put_huge_zero_folio();
>  }
> @@ -849,16 +855,34 @@ static inline void hugepage_exit_sysfs(struct kobject *hugepage_kobj)
>
>  static int __init thp_shrinker_init(void)
>  {
> -	huge_zero_folio_shrinker = shrinker_alloc(0, "thp-zero");
> -	if (!huge_zero_folio_shrinker)
> -		return -ENOMEM;
> -
>  	deferred_split_shrinker = shrinker_alloc(SHRINKER_NUMA_AWARE |
>  						 SHRINKER_MEMCG_AWARE |
>  						 SHRINKER_NONSLAB,
>  						 "thp-deferred_split");
> -	if (!deferred_split_shrinker) {
> -		shrinker_free(huge_zero_folio_shrinker);
> +	if (!deferred_split_shrinker)
> +		return -ENOMEM;
> +
> +	deferred_split_shrinker->count_objects = deferred_split_count;
> +	deferred_split_shrinker->scan_objects = deferred_split_scan;
> +	shrinker_register(deferred_split_shrinker);
> +
> +	if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) {
> +		/*
> +		 * Bump the reference of the huge_zero_folio and do not
> +		 * initialize the shrinker.
> +		 *
> +		 * huge_zero_folio will always be NULL on failure. We assume
> +		 * that get_huge_zero_folio() will most likely not fail as
> +		 * thp_shrinker_init() is invoked early on during boot.
> +		 */
> +		if (!get_huge_zero_folio())
> +			pr_warn("Allocating static huge zero folio failed\n");
> +		return 0;
> +	}
> +
> +	huge_zero_folio_shrinker = shrinker_alloc(0, "thp-zero");
> +	if (!huge_zero_folio_shrinker) {
> +		shrinker_free(deferred_split_shrinker);
>  		return -ENOMEM;
>  	}
>
> @@ -866,10 +890,6 @@ static int __init thp_shrinker_init(void)
>  	huge_zero_folio_shrinker->scan_objects = shrink_huge_zero_folio_scan;
>  	shrinker_register(huge_zero_folio_shrinker);
>
> -	deferred_split_shrinker->count_objects = deferred_split_count;
> -	deferred_split_shrinker->scan_objects = deferred_split_scan;
> -	shrinker_register(deferred_split_shrinker);
> -
>  	return 0;
>  }
>
> --
> 2.49.0
>