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Subject[RFC PATCH 10/24] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page
Date
When we allocate a hugetlb page from the buddy, we should free the
unused vmemmap pages associated with it. We can do that in the
prep_new_huge_page().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
include/linux/hugetlb.h | 21 ++++
mm/hugetlb.c | 231 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 252 insertions(+)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index ace304a6196c..2561af2ad901 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -601,6 +601,13 @@ static inline bool arch_vmemmap_support_huge_mapping(void)
}
#endif

+#ifndef vmemmap_pmd_huge
+static inline bool vmemmap_pmd_huge(pmd_t *pmd)
+{
+ return pmd_huge(*pmd);
+}
+#endif
+
#ifndef VMEMMAP_HPAGE_SHIFT
#define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
#endif
@@ -790,6 +797,15 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
}
#endif

+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+int handle_vmemmap_fault(unsigned long page);
+#else
+static inline int handle_vmemmap_fault(unsigned long page)
+{
+ return -EFAULT;
+}
+#endif
+
#else /* CONFIG_HUGETLB_PAGE */
struct hstate {};

@@ -943,6 +959,11 @@ 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 int handle_vmemmap_fault(unsigned long page)
+{
+ return -EFAULT;
+}
#endif /* CONFIG_HUGETLB_PAGE */

static inline spinlock_t *huge_pte_lock(struct hstate *h,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d6ae9b6876be..a628588a075a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1293,10 +1293,20 @@ static inline void destroy_compound_gigantic_page(struct page *page,
#endif

#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+#include <linux/bootmem_info.h>
+
#define RESERVE_VMEMMAP_NR 2U
+#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)

#define page_huge_pte(page) ((page)->pmd_huge_pte)

+#define vmemmap_hpage_addr_end(addr, end) \
+({ \
+ unsigned long __boundary; \
+ __boundary = ((addr) + VMEMMAP_HPAGE_SIZE) & VMEMMAP_HPAGE_MASK;\
+ (__boundary - 1 < (end) - 1) ? __boundary : (end); \
+})
+
static inline unsigned int nr_free_vmemmap(struct hstate *h)
{
return h->nr_free_vmemmap_pages;
@@ -1416,6 +1426,222 @@ static void __init hugetlb_vmemmap_init(struct hstate *h)
pr_info("HugeTLB: can free %d vmemmap pages for %s\n",
h->nr_free_vmemmap_pages, h->name);
}
+
+static inline spinlock_t *vmemmap_pmd_lockptr(pmd_t *pmd)
+{
+ static DEFINE_SPINLOCK(pgtable_lock);
+
+ return &pgtable_lock;
+}
+
+/*
+ * Walk a vmemmap address to the pmd it maps.
+ */
+static pmd_t *vmemmap_to_pmd(const void *page)
+{
+ unsigned long addr = (unsigned long)page;
+ pgd_t *pgd;
+ p4d_t *p4d;
+ pud_t *pud;
+ pmd_t *pmd;
+
+ if (addr < VMEMMAP_START || addr >= VMEMMAP_END)
+ return NULL;
+
+ pgd = pgd_offset_k(addr);
+ if (pgd_none(*pgd))
+ return NULL;
+ p4d = p4d_offset(pgd, addr);
+ if (p4d_none(*p4d))
+ return NULL;
+ pud = pud_offset(p4d, addr);
+
+ WARN_ON_ONCE(pud_bad(*pud));
+ if (pud_none(*pud) || pud_bad(*pud))
+ return NULL;
+ pmd = pmd_offset(pud, addr);
+
+ return pmd;
+}
+
+static inline int freed_vmemmap_hpage(struct page *page)
+{
+ return atomic_read(&page->_mapcount) + 1;
+}
+
+static inline int freed_vmemmap_hpage_inc(struct page *page)
+{
+ return atomic_inc_return_relaxed(&page->_mapcount) + 1;
+}
+
+static inline int freed_vmemmap_hpage_dec(struct page *page)
+{
+ return atomic_dec_return_relaxed(&page->_mapcount) + 1;
+}
+
+static inline void free_vmemmap_page_list(struct list_head *list)
+{
+ struct page *page, *next;
+
+ list_for_each_entry_safe(page, next, list, lru) {
+ list_del(&page->lru);
+ free_vmemmap_page(page);
+ }
+}
+
+static void __free_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
+ unsigned long start,
+ unsigned int nr_free,
+ struct list_head *free_pages)
+{
+ pte_t entry = mk_pte(reuse, PAGE_KERNEL);
+ unsigned long addr;
+ unsigned long end = start + (nr_free << PAGE_SHIFT);
+
+ for (addr = start; addr < end; addr += PAGE_SIZE, ptep++) {
+ struct page *page;
+ pte_t old = *ptep;
+
+ VM_WARN_ON(!pte_present(old));
+ page = pte_page(old);
+ list_add(&page->lru, free_pages);
+
+ set_pte_at(&init_mm, addr, ptep, entry);
+ }
+}
+
+static void __free_huge_page_pmd_vmemmap(struct hstate *h, pmd_t *pmd,
+ unsigned long addr,
+ struct list_head *free_pages)
+{
+ unsigned long next;
+ unsigned long start = addr + RESERVE_VMEMMAP_NR * PAGE_SIZE;
+ unsigned long end = addr + nr_vmemmap_size(h);
+ struct page *reuse = NULL;
+
+ addr = start;
+ do {
+ unsigned int nr_pages;
+ pte_t *ptep;
+
+ ptep = pte_offset_kernel(pmd, addr);
+ if (!reuse)
+ reuse = pte_page(ptep[-1]);
+
+ next = vmemmap_hpage_addr_end(addr, end);
+ nr_pages = (next - addr) >> PAGE_SHIFT;
+ __free_huge_page_pte_vmemmap(reuse, ptep, addr, nr_pages,
+ free_pages);
+ } while (pmd++, addr = next, addr != end);
+
+ flush_tlb_kernel_range(start, end);
+}
+
+static void split_vmemmap_pmd(pmd_t *pmd, pte_t *pte_p, unsigned long addr)
+{
+ struct mm_struct *mm = &init_mm;
+ struct page *page;
+ pmd_t old_pmd, _pmd;
+ int i;
+
+ /*
+ * Up to this point the pmd is present and huge and userland has the
+ * whole access to the hugepage during the split (which happens in
+ * place). If we overwrite the pmd with the not-huge version pointing
+ * to the pte here (which of course we could if all CPUs were bug
+ * free), userland could trigger a small page size TLB miss on the
+ * small sized TLB while the hugepage TLB entry is still established in
+ * the huge TLB. Some CPU doesn't like that.
+ *
+ * See http://support.amd.com/us/Processor_TechDocs/41322.pdf, Erratum
+ * 383 on page 93. Intel should be safe but is also warns that it's
+ * only safe if the permission and cache attributes of the two entries
+ * loaded in the two TLB is identical (which should be the case here).
+ *
+ * So it is generally safer to never allow small and huge TLB entries
+ * for the same virtual address to be loaded simultaneously. But here
+ * we should not set pmd non-present first and flush TLB. Because if
+ * we do that(maybe trriger IPI to other CPUs to flush TLB), we may be
+ * deadlocked. So we have to break the above rules. Be careful, Let us
+ * suppose all CPUs are bug free, otherwise, we should not enable the
+ * feature of freeing unused vmemmap pages on the bug CPU.
+ *
+ * Why we should not set pmd non-present first? Here we already hold
+ * the vmemmap pgtable spinlock on CPU1 and set pmd non-present. If
+ * CPU0 access the struct page with irqs disabled and the vmemmap
+ * pgtable lock is held by CPU1. In this case, the CPU0 can not handle
+ * the IPI interrupt to flush TLB because of the disabling of irqs.
+ * Then we can deadlock. In order to avoid this issue, we do not set
+ * pmd non-present.
+ *
+ * The deadlock scene is shown below.
+ *
+ * CPU0: CPU1:
+ * disable irqs hold the vmemmap pgtable lock
+ * set pmd non-present
+ * read/write `struct page`(page fault)
+ * jump to handle_vmemmap_fault
+ * spin for vmemmap pgtable lock
+ * flush_tlb(send IPI to CPU0)
+ * set new pmd(small page)
+ */
+ old_pmd = READ_ONCE(*pmd);
+ page = pmd_page(old_pmd);
+ pmd_populate_kernel(mm, &_pmd, pte_p);
+
+ for (i = 0; i < VMEMMAP_HPAGE_NR; i++, addr += PAGE_SIZE) {
+ pte_t entry, *pte;
+
+ entry = mk_pte(page + i, PAGE_KERNEL);
+ pte = pte_offset_kernel(&_pmd, addr);
+ VM_BUG_ON(!pte_none(*pte));
+ set_pte_at(mm, addr, pte, entry);
+ }
+
+ /* make pte visible before pmd */
+ smp_wmb();
+ pmd_populate_kernel(mm, pmd, pte_p);
+}
+
+static void split_vmemmap_huge_page(struct page *head, pmd_t *pmd)
+{
+ pte_t *pte_p;
+ unsigned long start = (unsigned long)head & VMEMMAP_HPAGE_MASK;
+ unsigned long addr = start;
+
+ while ((pte_p = vmemmap_pgtable_withdraw(head))) {
+ VM_BUG_ON(freed_vmemmap_hpage(virt_to_page(pte_p)));
+ split_vmemmap_pmd(pmd++, pte_p, addr);
+ addr += VMEMMAP_HPAGE_SIZE;
+ }
+
+ flush_tlb_kernel_range(start, addr);
+}
+
+static void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+ pmd_t *pmd;
+ spinlock_t *ptl;
+ LIST_HEAD(free_pages);
+
+ if (!nr_free_vmemmap(h))
+ return;
+
+ pmd = vmemmap_to_pmd(head);
+ ptl = vmemmap_pmd_lockptr(pmd);
+
+ spin_lock(ptl);
+ if (vmemmap_pmd_huge(pmd)) {
+ VM_BUG_ON(!nr_pgtable(h));
+ split_vmemmap_huge_page(head, pmd);
+ }
+
+ __free_huge_page_pmd_vmemmap(h, pmd, (unsigned long)head, &free_pages);
+ freed_vmemmap_hpage_inc(pmd_page(*pmd));
+ spin_unlock(ptl);
+
+ free_vmemmap_page_list(&free_pages);
+}
#else
static inline void hugetlb_vmemmap_init(struct hstate *h)
{
@@ -1429,6 +1655,10 @@ static inline int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
{
}
+
+static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+}
#endif

static void update_and_free_page(struct hstate *h, struct page *page)
@@ -1637,6 +1867,7 @@ void free_huge_page(struct page *page)

static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
{
+ free_huge_page_vmemmap(h, page);
/* Must be called before the initialization of @page->lru */
vmemmap_pgtable_free(h, page);

--
2.20.1
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