mm: add persistent huge zero folio

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/

Link: https://lkml.kernel.org/r/20250811084113.647267-4-kernel@pankajraghav.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Co-developed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Luis Chamberalin <mcgrof@kernel.org>
Cc: Mariano Pache <npache@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Kiryl Shutsemau <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 7748489fde1b7af59b3f926233c98e27d26dbac6..bd547857c6c1a8e4dd56849a644f67b82f15166c 100644 (file)
--- 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 59a04d0b2e272e33551643ce66db7d9ddc4128f3..4108bcd96784860481e6c2d0e82cf45ad9ffbcb3 100644 (file)
--- 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. 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's 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 2801ce9bbde932f8535bc6e4113a0ac5dda3a450..b8bb078a1a348b9a9a57929d847db396189dd808 100644 (file)
--- 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 persistent 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;
 }