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>
---
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
+ 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.
+
+ With this option enabled, the huge zero folio is allocated
+ once and never freed. One full huge page worth of memory shall
+ be used.
+
+ 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;
}
--
2.49.0On 8/8/25 14:11, 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> > --- > include/linux/huge_mm.h | 16 ++++++++++++++++ > mm/Kconfig | 16 ++++++++++++++++ > mm/huge_memory.c | 40 ++++++++++++++++++++++++++++++---------- > 3 files changed, 62 insertions(+), 10 deletions(-) > As mentioned, I really would like to have a kernel commandline parameter for disabling huge zero folio. Otherwise: Reviewed-by: Hannes Reinecke <hare@suse.de> Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich
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
>
> This is much nicer and now _super_ simple, I like it. Thanks to you and David :) > > A few nits below but generally: > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> Thanks. > > > --- > > include/linux/huge_mm.h | 16 ++++++++++++++++ > > mm/Kconfig | 16 ++++++++++++++++ > > mm/huge_memory.c | 40 ++++++++++++++++++++++++++++++---------- > > 3 files changed, 62 insertions(+), 10 deletions(-) > > > > 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 :) I already brought this topic once during THP cabal. I am thinking of submitting a talk about this topic for LPC Memory Management MC. > > > + 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 > Thanks for the comments. I will make those changes and send a new version. -- Pankaj
> + if (!get_huge_zero_folio())
> + pr_warn("Allocating static huge zero folio failed\n");
Oops, forgot to do s/static/persistent/ here.
I can fold this in the next version after receiving the comments.
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