[RFC] Demand faulting for large pages

[Adam Litke <agl@us.ibm.com>]

Below is a patch to implement demand faulting for huge pages.  The main
motivation for changing from prefaulting to demand faulting is so that
huge page allocations can follow the NUMA API.  Currently, huge pages
are allocated round-robin from all NUMA nodes.   

The default behavior in SLES9 for i386 is to use demand faulting with
NUMA policy-aware allocations.  To my knowledge, this continues to work
well in practice.  Thanks to consolidated hugetlb code, switching the
behavior requires changing only one fault handler.  The bulk of the
patch just moves the logic from hugelb_prefault() to
hugetlb_pte_fault().

Diffed against 2.6.13-rc4-git4

Signed-off-by: Adam Litke <agl@us.ibm.com>

 fs/hugetlbfs/inode.c    |    5 -
 include/linux/hugetlb.h |    2 
 mm/hugetlb.c            |  140 +++++++++++++++++++++++++++---------------------
 mm/memory.c             |    7 --
 4 files changed, 83 insertions(+), 71 deletions(-)
diff -upN reference/fs/hugetlbfs/inode.c current/fs/hugetlbfs/inode.c
--- reference/fs/hugetlbfs/inode.c
+++ current/fs/hugetlbfs/inode.c
@@ -79,10 +79,7 @@ static int hugetlbfs_file_mmap(struct fi
 	if (!(vma->vm_flags & VM_WRITE) && len > inode->i_size)
 		goto out;
 
-	ret = hugetlb_prefault(mapping, vma);
-	if (ret)
-		goto out;
-
+	ret = 0;
 	if (inode->i_size < len)
 		inode->i_size = len;
 out:
diff -upN reference/include/linux/hugetlb.h current/include/linux/hugetlb.h
--- reference/include/linux/hugetlb.h
+++ current/include/linux/hugetlb.h
@@ -25,6 +25,8 @@ int is_hugepage_mem_enough(size_t);
 unsigned long hugetlb_total_pages(void);
 struct page *alloc_huge_page(void);
 void free_huge_page(struct page *);
+int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct * vma,
+			unsigned long address, int write_access);
 
 extern unsigned long max_huge_pages;
 extern const unsigned long hugetlb_zero, hugetlb_infinity;
diff -upN reference/mm/hugetlb.c current/mm/hugetlb.c
--- reference/mm/hugetlb.c
+++ current/mm/hugetlb.c
@@ -277,18 +277,20 @@ int copy_hugetlb_page_range(struct mm_st
 	unsigned long addr = vma->vm_start;
 	unsigned long end = vma->vm_end;
 
-	while (addr < end) {
+	for (; addr < end; addr += HPAGE_SIZE) {
+		src_pte = huge_pte_offset(src, addr);
+		if (!src_pte || pte_none(*src_pte))
+			continue;
+		
 		dst_pte = huge_pte_alloc(dst, addr);
 		if (!dst_pte)
 			goto nomem;
-		src_pte = huge_pte_offset(src, addr);
-		BUG_ON(!src_pte || pte_none(*src_pte)); /* prefaulted */
+		BUG_ON(!src_pte);
 		entry = *src_pte;
 		ptepage = pte_page(entry);
 		get_page(ptepage);
 		add_mm_counter(dst, rss, HPAGE_SIZE / PAGE_SIZE);
 		set_huge_pte_at(dst, addr, dst_pte, entry);
-		addr += HPAGE_SIZE;
 	}
 	return 0;
 
@@ -329,63 +331,6 @@ void zap_hugepage_range(struct vm_area_s
 	spin_unlock(&mm->page_table_lock);
 }
 
-int hugetlb_prefault(struct address_space *mapping, struct vm_area_struct *vma)
-{
-	struct mm_struct *mm = current->mm;
-	unsigned long addr;
-	int ret = 0;
-
-	WARN_ON(!is_vm_hugetlb_page(vma));
-	BUG_ON(vma->vm_start & ~HPAGE_MASK);
-	BUG_ON(vma->vm_end & ~HPAGE_MASK);
-
-	hugetlb_prefault_arch_hook(mm);
-
-	spin_lock(&mm->page_table_lock);
-	for (addr = vma->vm_start; addr < vma->vm_end; addr += HPAGE_SIZE) {
-		unsigned long idx;
-		pte_t *pte = huge_pte_alloc(mm, addr);
-		struct page *page;
-
-		if (!pte) {
-			ret = -ENOMEM;
-			goto out;
-		}
-		if (! pte_none(*pte))
-			hugetlb_clean_stale_pgtable(pte);
-
-		idx = ((addr - vma->vm_start) >> HPAGE_SHIFT)
-			+ (vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT));
-		page = find_get_page(mapping, idx);
-		if (!page) {
-			/* charge the fs quota first */
-			if (hugetlb_get_quota(mapping)) {
-				ret = -ENOMEM;
-				goto out;
-			}
-			page = alloc_huge_page();
-			if (!page) {
-				hugetlb_put_quota(mapping);
-				ret = -ENOMEM;
-				goto out;
-			}
-			ret = add_to_page_cache(page, mapping, idx, GFP_ATOMIC);
-			if (! ret) {
-				unlock_page(page);
-			} else {
-				hugetlb_put_quota(mapping);
-				free_huge_page(page);
-				goto out;
-			}
-		}
-		add_mm_counter(mm, rss, HPAGE_SIZE / PAGE_SIZE);
-		set_huge_pte_at(mm, addr, pte, make_huge_pte(vma, page));
-	}
-out:
-	spin_unlock(&mm->page_table_lock);
-	return ret;
-}
-
 int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			struct page **pages, struct vm_area_struct **vmas,
 			unsigned long *position, int *length, int i)
@@ -433,3 +378,76 @@ int follow_hugetlb_page(struct mm_struct
 
 	return i;
 }
+
+int hugetlb_pte_fault(struct mm_struct *mm, struct vm_area_struct *vma,
+			unsigned long address, int write_access)
+{
+	int ret = VM_FAULT_MINOR;
+	unsigned long idx;
+	pte_t *pte;
+	struct page *page;
+	struct address_space *mapping;
+
+	WARN_ON(!is_vm_hugetlb_page(vma));
+	BUG_ON(vma->vm_start & ~HPAGE_MASK);
+	BUG_ON(vma->vm_end & ~HPAGE_MASK);
+	BUG_ON(!vma->vm_file);
+
+	pte = huge_pte_alloc(mm, address);
+	if (!pte) {
+		ret = VM_FAULT_SIGBUS;
+		goto out;
+	}
+	if (! pte_none(*pte))
+		goto flush;
+
+	mapping = vma->vm_file->f_mapping;
+	idx = ((address - vma->vm_start) >> HPAGE_SHIFT)
+		+ (vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT));
+retry:
+	page = find_get_page(mapping, idx);
+	if (!page) {
+		/* charge the fs quota first */
+		if (hugetlb_get_quota(mapping)) {
+			ret = VM_FAULT_SIGBUS;
+			goto out;
+		}
+		page = alloc_huge_page();
+		if (!page) {
+			hugetlb_put_quota(mapping);
+			ret = VM_FAULT_SIGBUS;
+			goto out;
+		}
+		if(add_to_page_cache(page, mapping, idx, GFP_ATOMIC)) {
+			put_page(page);
+			goto retry;
+		}
+		unlock_page(page);
+	}
+	add_mm_counter(mm, rss, HPAGE_SIZE / PAGE_SIZE);
+	set_huge_pte_at(mm, address, pte, make_huge_pte(vma, page));
+flush:
+	flush_tlb_page(vma, address);
+out:
+	return ret;
+}
+
+int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
+			unsigned long address, int write_access)
+{
+	pte_t *ptep;
+	int rc = VM_FAULT_SIGBUS;
+
+	spin_lock(&mm->page_table_lock);
+
+	ptep = huge_pte_alloc(mm, address & HPAGE_MASK);
+	if (! ptep) {
+		BUG();
+		goto out;
+	}
+	if (pte_none(*ptep))
+		rc = hugetlb_pte_fault(mm, vma, address, write_access);
+out:
+	spin_unlock(&mm->page_table_lock);
+	return rc;
+}
diff -upN reference/mm/memory.c current/mm/memory.c
--- reference/mm/memory.c
+++ current/mm/memory.c
@@ -933,11 +933,6 @@ int get_user_pages(struct task_struct *t
 				|| !(flags & vma->vm_flags))
 			return i ? : -EFAULT;
 
-		if (is_vm_hugetlb_page(vma)) {
-			i = follow_hugetlb_page(mm, vma, pages, vmas,
-						&start, &len, i);
-			continue;
-		}
 		spin_lock(&mm->page_table_lock);
 		do {
 			struct page *page;
@@ -2024,7 +2019,7 @@ int handle_mm_fault(struct mm_struct *mm
 	inc_page_state(pgfault);
 
 	if (is_vm_hugetlb_page(vma))
-		return VM_FAULT_SIGBUS;	/* mapping truncation does this. */
+		return hugetlb_fault(mm, vma, address, write_access);
 
 	/*
 	 * We need the page table lock to synchronize with kswapd

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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