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Subject[RFC PATCH v2 03/30] mm: thp: use single linked list for THP page table page deposit.
Date
From: Zi Yan <ziy@nvidia.com>

The old design uses the double linked list page->lru to chain all
deposited page table pages when creating a THP and page->pmd_huge_pte
to point to the first page of the list. As the second pointer in
page->lru overlaps with page->pmd_huge_pte, the design prevents
multi-level page table page deposit, which is useful for PUD and higher
level THPs.

The new design uses single linked list, where deposit_head points to
a single linked list of deposited pages and deposit_node can be used to
deposit the page itself to another list. For example, this allows us to
have one PUD page points to a list of PMD pages, each of which points
a list of PTE pages to support PUD level THP.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
include/linux/mm.h | 9 +++++----
include/linux/mm_types.h | 8 +++++---
kernel/fork.c | 4 ++--
mm/pgtable-generic.c | 15 +++++----------
4 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 17e712207d74..01b62da34794 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -10,6 +10,7 @@
#include <linux/gfp.h>
#include <linux/bug.h>
#include <linux/list.h>
+#include <linux/llist.h>
#include <linux/mmzone.h>
#include <linux/rbtree.h>
#include <linux/atomic.h>
@@ -2249,7 +2250,7 @@ static inline spinlock_t *pmd_lockptr(struct mm_struct *mm, pmd_t *pmd)
static inline bool pmd_ptlock_init(struct page *page)
{
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- page->pmd_huge_pte = NULL;
+ init_llist_head(&page->deposit_head);
#endif
return ptlock_init(page);
}
@@ -2257,12 +2258,12 @@ static inline bool pmd_ptlock_init(struct page *page)
static inline void pmd_ptlock_free(struct page *page)
{
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- VM_BUG_ON_PAGE(page->pmd_huge_pte, page);
+ VM_BUG_ON_PAGE(!llist_empty(&page->deposit_head), page);
#endif
ptlock_free(page);
}

-#define pmd_huge_pte(mm, pmd) (pmd_to_page(pmd)->pmd_huge_pte)
+#define huge_pmd_deposit_head(mm, pmd) (pmd_to_page(pmd)->deposit_head)

#else

@@ -2274,7 +2275,7 @@ static inline spinlock_t *pmd_lockptr(struct mm_struct *mm, pmd_t *pmd)
static inline bool pmd_ptlock_init(struct page *page) { return true; }
static inline void pmd_ptlock_free(struct page *page) {}

-#define pmd_huge_pte(mm, pmd) ((mm)->pmd_huge_pte)
+#define huge_pmd_deposit_head(mm, pmd) ((mm)->deposit_head_pmd)

#endif

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 496c3ff97cce..be842926577a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -6,6 +6,7 @@

#include <linux/auxvec.h>
#include <linux/list.h>
+#include <linux/llist.h>
#include <linux/spinlock.h>
#include <linux/rbtree.h>
#include <linux/rwsem.h>
@@ -143,8 +144,8 @@ struct page {
struct list_head deferred_list;
};
struct { /* Page table pages */
- unsigned long _pt_pad_1; /* compound_head */
- pgtable_t pmd_huge_pte; /* protected by page->ptl */
+ struct llist_head deposit_head; /* pgtable deposit list head */
+ struct llist_node deposit_node; /* pgtable deposit list node */
unsigned long _pt_pad_2; /* mapping */
union {
struct mm_struct *pt_mm; /* x86 pgds only */
@@ -511,7 +512,8 @@ struct mm_struct {
struct mmu_notifier_subscriptions *notifier_subscriptions;
#endif
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
- pgtable_t pmd_huge_pte; /* protected by page_table_lock */
+ /* pgtable deposit list head, protected by page_table_lock */
+ struct llist_head deposit_head_pmd;
#endif
#ifdef CONFIG_NUMA_BALANCING
/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 138cd6ca50da..9c8e880538de 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -661,7 +661,7 @@ static void check_mm(struct mm_struct *mm)
mm_pgtables_bytes(mm));

#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
- VM_BUG_ON_MM(mm->pmd_huge_pte, mm);
+ VM_BUG_ON_MM(!llist_empty(&mm->deposit_head_pmd), mm);
#endif
}

@@ -1022,7 +1022,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mmu_notifier_subscriptions_init(mm);
init_tlb_flush_pending(mm);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
- mm->pmd_huge_pte = NULL;
+ init_llist_head(&mm->deposit_head_pmd);
#endif
mm_init_uprobes_state(mm);

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 9578db83e312..dbb0154165f1 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -164,11 +164,7 @@ void pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp,
assert_spin_locked(pmd_lockptr(mm, pmdp));

/* FIFO */
- if (!pmd_huge_pte(mm, pmdp))
- INIT_LIST_HEAD(&pgtable->lru);
- else
- list_add(&pgtable->lru, &pmd_huge_pte(mm, pmdp)->lru);
- pmd_huge_pte(mm, pmdp) = pgtable;
+ llist_add(&pgtable->deposit_node, &huge_pmd_deposit_head(mm, pmdp));
}
#endif

@@ -180,12 +176,11 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)

assert_spin_locked(pmd_lockptr(mm, pmdp));

+ /* only withdraw from a non empty list */
+ VM_BUG_ON(llist_empty(&huge_pmd_deposit_head(mm, pmdp)));
/* FIFO */
- pgtable = pmd_huge_pte(mm, pmdp);
- pmd_huge_pte(mm, pmdp) = list_first_entry_or_null(&pgtable->lru,
- struct page, lru);
- if (pmd_huge_pte(mm, pmdp))
- list_del(&pgtable->lru);
+ pgtable = llist_entry(llist_del_first(&huge_pmd_deposit_head(mm, pmdp)),
+ struct page, deposit_node);
return pgtable;
}
#endif
--
2.28.0
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