Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
freeing unused vmemmap pages associated with each hugetlb page on boot.

We disables PMD mapping of vmemmap pages for x86-64 arch when this
feature is enabled. Because vmemmap_remap_free() depends on vmemmap
being base page mapped.

Signed-off-by: Muchun Song <songmuc...@bytedance.com>
Reviewed-by: Oscar Salvador <osalva...@suse.de>
Reviewed-by: Barry Song <song.bao....@hisilicon.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
 Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
 arch/x86/mm/init_64.c                           |  8 ++++++--
 include/linux/hugetlb.h                         | 19 +++++++++++++++++++
 mm/hugetlb_vmemmap.c                            | 22 ++++++++++++++++++++++
 5 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 5adf1e57e932..7db2591f3ad3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1577,6 +1577,20 @@
                        Documentation/admin-guide/mm/hugetlbpage.rst.
                        Format: size[KMG]
 
+       hugetlb_free_vmemmap=
+                       [KNL] When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set,
+                       this controls freeing unused vmemmap pages associated
+                       with each HugeTLB page. When this option is enabled,
+                       we disable PMD/huge page mapping of vmemmap pages which
+                       increase page table pages. So if a user/sysadmin only
+                       uses a small number of HugeTLB pages (as a percentage
+                       of system memory), they could end up using more memory
+                       with hugetlb_free_vmemmap on as opposed to off.
+                       Format: { on | off (default) }
+
+                       on:  enable the feature
+                       off: disable the feature
+
        hung_task_panic=
                        [KNL] Should the hung task detector generate panics.
                        Format: 0 | 1
diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst 
b/Documentation/admin-guide/mm/hugetlbpage.rst
index f7b1c7462991..3a23c2377acc 100644
--- a/Documentation/admin-guide/mm/hugetlbpage.rst
+++ b/Documentation/admin-guide/mm/hugetlbpage.rst
@@ -145,6 +145,9 @@ default_hugepagesz
 
        will all result in 256 2M huge pages being allocated.  Valid default
        huge page size is architecture dependent.
+hugetlb_free_vmemmap
+       When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set, this enables freeing
+       unused vmemmap pages associated with each HugeTLB page.
 
 When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
 indicates the current number of pre-allocated huge pages of the default size.
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0435bee2e172..39f88c5faadc 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -34,6 +34,7 @@
 #include <linux/gfp.h>
 #include <linux/kcore.h>
 #include <linux/bootmem_info.h>
+#include <linux/hugetlb.h>
 
 #include <asm/processor.h>
 #include <asm/bios_ebda.h>
@@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, 
unsigned long end, int node,
 {
        int err;
 
-       if (end - start < PAGES_PER_SECTION * sizeof(struct page))
+       if ((is_hugetlb_free_vmemmap_enabled()  && !altmap) ||
+           end - start < PAGES_PER_SECTION * sizeof(struct page))
                err = vmemmap_populate_basepages(start, end, node, NULL);
        else if (boot_cpu_has(X86_FEATURE_PSE))
                err = vmemmap_populate_hugepages(start, end, node, altmap);
@@ -1585,6 +1587,8 @@ void register_page_bootmem_memmap(unsigned long 
section_nr,
        pmd_t *pmd;
        unsigned int nr_pmd_pages;
        struct page *page;
+       bool base_mapping = !boot_cpu_has(X86_FEATURE_PSE) ||
+                           is_hugetlb_free_vmemmap_enabled();
 
        for (; addr < end; addr = next) {
                pte_t *pte = NULL;
@@ -1610,7 +1614,7 @@ void register_page_bootmem_memmap(unsigned long 
section_nr,
                }
                get_page_bootmem(section_nr, pud_page(*pud), MIX_SECTION_INFO);
 
-               if (!boot_cpu_has(X86_FEATURE_PSE)) {
+               if (base_mapping) {
                        next = (addr + PAGE_SIZE) & PAGE_MASK;
                        pmd = pmd_offset(pud, addr);
                        if (pmd_none(*pmd))
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 37fd248ce271..ad249e56ac49 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -854,6 +854,20 @@ static inline void huge_ptep_modify_prot_commit(struct 
vm_area_struct *vma,
 
 void set_page_huge_active(struct page *page);
 
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+extern bool hugetlb_free_vmemmap_enabled;
+
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+       return hugetlb_free_vmemmap_enabled;
+}
+#else
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+       return false;
+}
+#endif
+
 #else  /* CONFIG_HUGETLB_PAGE */
 struct hstate {};
 
@@ -1007,6 +1021,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct 
*mm, unsigned long addr
                                        pte_t *ptep, pte_t pte, unsigned long 
sz)
 {
 }
+
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+       return false;
+}
 #endif /* CONFIG_HUGETLB_PAGE */
 
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 0bd6b8d7282d..224a3cb69bf9 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -183,6 +183,28 @@
 #define RESERVE_VMEMMAP_NR             2U
 #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
+bool hugetlb_free_vmemmap_enabled;
+
+static int __init early_hugetlb_free_vmemmap_param(char *buf)
+{
+       /* We cannot optimize if a "struct page" crosses page boundaries. */
+       if ((!is_power_of_2(sizeof(struct page)))) {
+               pr_warn("cannot free vmemmap pages because \"struct page\" 
crosses page boundaries\n");
+               return 0;
+       }
+
+       if (!buf)
+               return -EINVAL;
+
+       if (!strcmp(buf, "on"))
+               hugetlb_free_vmemmap_enabled = true;
+       else if (strcmp(buf, "off"))
+               return -EINVAL;
+
+       return 0;
+}
+early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
+
 /*
  * How many vmemmap pages associated with a HugeTLB page that can be freed
  * to the buddy allocator.
-- 
2.11.0

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