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Subject[PATCH, RFC 2/2] Implement sharing/unsharing of PMDs for FS/DAX
Date
This is based on (but somewhat different from) what hugetlbfs
does to share/unshare page tables.

Signed-off-by: Larry Bassel <larry.bassel@oracle.com>
---
include/linux/hugetlb.h | 4 ++
mm/huge_memory.c | 32 ++++++++++++++
mm/hugetlb.c | 21 ++++++++--
mm/memory.c | 108 +++++++++++++++++++++++++++++++++++++++++++++++-
4 files changed, 160 insertions(+), 5 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 11943b6..9ed9542 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -142,6 +142,10 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr, pte_t *ptep);
void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
unsigned long *start, unsigned long *end);
+unsigned long page_table_shareable(struct vm_area_struct *svma,
+ struct vm_area_struct *vma,
+ unsigned long addr, pgoff_t idx);
+bool vma_shareable(struct vm_area_struct *vma, unsigned long addr);
struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
int write);
struct page *follow_huge_pd(struct vm_area_struct *vma,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b6a34b3..e1627c3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1747,6 +1747,33 @@ static inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd)
mm_dec_nr_ptes(mm);
}

+#ifdef CONFIG_MAY_SHARE_FSDAX_PMD
+static int unshare_huge_pmd(struct mm_struct *mm, unsigned long addr,
+ pmd_t *pmdp)
+{
+ pgd_t *pgd = pgd_offset(mm, addr);
+ p4d_t *p4d = p4d_offset(pgd, addr);
+ pud_t *pud = pud_offset(p4d, addr);
+
+ WARN_ON(page_count(virt_to_page(pmdp)) == 0);
+ if (page_count(virt_to_page(pmdp)) == 1)
+ return 0;
+
+ pud_clear(pud);
+ put_page(virt_to_page(pmdp));
+ mm_dec_nr_pmds(mm);
+ return 1;
+}
+
+#else
+static int unshare_huge_pmd(struct mm_struct *mm, unsigned long addr,
+ pmd_t *pmdp)
+{
+ return 0;
+}
+
+#endif
+
int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
pmd_t *pmd, unsigned long addr)
{
@@ -1764,6 +1791,11 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
* pgtable_trans_huge_withdraw after finishing pmdp related
* operations.
*/
+ if (unshare_huge_pmd(vma->vm_mm, addr, pmd)) {
+ spin_unlock(ptl);
+ return 1;
+ }
+
orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
tlb->fullmm);
tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 641cedf..919a290 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4594,9 +4594,9 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
}

#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
-static unsigned long page_table_shareable(struct vm_area_struct *svma,
- struct vm_area_struct *vma,
- unsigned long addr, pgoff_t idx)
+unsigned long page_table_shareable(struct vm_area_struct *svma,
+ struct vm_area_struct *vma,
+ unsigned long addr, pgoff_t idx)
{
unsigned long saddr = ((idx - svma->vm_pgoff) << PAGE_SHIFT) +
svma->vm_start;
@@ -4619,7 +4619,7 @@ static unsigned long page_table_shareable(struct vm_area_struct *svma,
return saddr;
}

-static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
+bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
{
unsigned long base = addr & PUD_MASK;
unsigned long end = base + PUD_SIZE;
@@ -4763,6 +4763,19 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
unsigned long *start, unsigned long *end)
{
}
+
+unsigned long page_table_shareable(struct vm_area_struct *svma,
+ struct vm_area_struct *vma,
+ unsigned long addr, pgoff_t idx)
+{
+ return 0;
+}
+
+bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
+{
+ return false;
+}
+
#define want_pmd_share() (0)
#endif /* CONFIG_ARCH_WANT_HUGE_PMD_SHARE */

diff --git a/mm/memory.c b/mm/memory.c
index f7d962d..4c1814c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3845,6 +3845,109 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
return 0;
}

+#ifdef CONFIG_MAY_SHARE_FSDAX_PMD
+static pmd_t *huge_pmd_offset(struct mm_struct *mm,
+ unsigned long addr, unsigned long sz)
+{
+ pgd_t *pgd;
+ p4d_t *p4d;
+ pud_t *pud;
+ pmd_t *pmd;
+
+ pgd = pgd_offset(mm, addr);
+ if (!pgd_present(*pgd))
+ return NULL;
+ p4d = p4d_offset(pgd, addr);
+ if (!p4d_present(*p4d))
+ return NULL;
+
+ pud = pud_offset(p4d, addr);
+ if (sz != PUD_SIZE && pud_none(*pud))
+ return NULL;
+ /* hugepage or swap? */
+ if (pud_huge(*pud) || !pud_present(*pud))
+ return (pmd_t *)pud;
+
+ pmd = pmd_offset(pud, addr);
+ if (sz != PMD_SIZE && pmd_none(*pmd))
+ return NULL;
+ /* hugepage or swap? */
+ if (pmd_huge(*pmd) || !pmd_present(*pmd))
+ return pmd;
+
+ return NULL;
+}
+
+static pmd_t *pmd_share(struct mm_struct *mm, pud_t *pud, unsigned long addr)
+{
+ struct vm_area_struct *vma = find_vma(mm, addr);
+ struct address_space *mapping = vma->vm_file->f_mapping;
+ pgoff_t idx = ((addr - vma->vm_start) >> PAGE_SHIFT) +
+ vma->vm_pgoff;
+ struct vm_area_struct *svma;
+ unsigned long saddr;
+ pmd_t *spmd = NULL;
+ pmd_t *pmd;
+ spinlock_t *ptl;
+
+ if (!vma_shareable(vma, addr))
+ return pmd_alloc(mm, pud, addr);
+
+ i_mmap_lock_write(mapping);
+
+ vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
+ if (svma == vma)
+ continue;
+
+ saddr = page_table_shareable(svma, vma, addr, idx);
+ if (saddr) {
+ spmd = huge_pmd_offset(svma->vm_mm, saddr,
+ vma_mmu_pagesize(svma));
+ if (spmd) {
+ get_page(virt_to_page(spmd));
+ break;
+ }
+ }
+ }
+
+ if (!spmd)
+ goto out;
+
+ ptl = pmd_lockptr(mm, spmd);
+ spin_lock(ptl);
+
+ if (pud_none(*pud)) {
+ pud_populate(mm, pud,
+ (pmd_t *)((unsigned long)spmd & PAGE_MASK));
+ mm_inc_nr_pmds(mm);
+ } else {
+ put_page(virt_to_page(spmd));
+ }
+ spin_unlock(ptl);
+out:
+ pmd = pmd_alloc(mm, pud, addr);
+ i_mmap_unlock_write(mapping);
+ return pmd;
+}
+
+static bool may_share_pmd(struct vm_area_struct *vma)
+{
+ if (vma_is_fsdax(vma))
+ return true;
+ return false;
+}
+#else
+static pmd_t *pmd_share(struct mm_struct *mm, pud_t *pud, unsigned long addr)
+{
+ return pmd_alloc(mm, pud, addr);
+}
+
+static bool may_share_pmd(struct vm_area_struct *vma)
+{
+ return false;
+}
+#endif
+
/*
* By the time we get here, we already hold the mm semaphore
*
@@ -3898,7 +4001,10 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
}
}

- vmf.pmd = pmd_alloc(mm, vmf.pud, address);
+ if (unlikely(may_share_pmd(vma)))
+ vmf.pmd = pmd_share(mm, vmf.pud, address);
+ else
+ vmf.pmd = pmd_alloc(mm, vmf.pud, address);
if (!vmf.pmd)
return VM_FAULT_OOM;
if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
--
1.8.3.1
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