Re: [PATCH v5 3/9] mm/mremap: Use pmd/pud_poplulate to update page table entries

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:

> On 5/20/21 6:16 PM, Peter Xu wrote:
>> On Thu, May 20, 2021 at 01:56:54PM +0530, Aneesh Kumar K.V wrote:
>>>> This seems to work at least for my userfaultfd test on shmem, however I don't
>>>> fully understand the commit message [1] on: How do we guarantee we're not
>>>> moving a thp pte?
>>>>
>>>
>>> move_page_tables() checks for pmd_trans_huge() and ends up calling
>>> move_huge_pmd if it is a THP entry.
>> 
>> Sorry to be unclear: what if a huge pud thp?
>> 
>
> I am still checking. Looking at the code before commit 
> c49dd340180260c6239e453263a9a244da9a7c85, I don't see kernel handling 
> huge pud thp. I haven't studied huge pud thp enough to understand 
> whether c49dd340180260c6239e453263a9a244da9a7c85 intent to add that 
> support.
>
> We can do a move_huge_pud() like we do for huge pmd thp. But I am not 
> sure whether we handle those VMA's earlier and restrict mremap on them?

something like this? (not even compile tested). I am still not sure
whether this is really needed or we handle DAX VMA's in some other form.

diff --git a/mm/mremap.c b/mm/mremap.c
index 47c255b60150..037a7bd311f1 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -324,10 +324,51 @@ static inline bool move_normal_pud(struct vm_area_struct *vma,
 }
 #endif
 
+
+static bool move_huge_pud(struct vm_area_struct *vma, unsigned long old_addr,
+			  unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
+{
+	spinlock_t *old_ptl, *new_ptl;
+	struct mm_struct *mm = vma->vm_mm;
+	pud_t pud;
+
+	/*
+	 * The destination pud shouldn't be established, free_pgtables()
+	 * should have released it.
+	 */
+	if (WARN_ON_ONCE(!pud_none(*new_pud)))
+		return false;
+
+	/*
+	 * We don't have to worry about the ordering of src and dst
+	 * ptlocks because exclusive mmap_lock prevents deadlock.
+	 */
+	old_ptl = pud_lock(vma->vm_mm, old_pud);
+	new_ptl = pud_lockptr(mm, new_pud);
+	if (new_ptl != old_ptl)
+		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+	/* Clear the pud */
+	pud = *old_pud;
+	pud_clear(old_pud);
+
+	VM_BUG_ON(!pud_none(*new_pud));
+
+	/* Set the new pud */
+	set_pud_at(mm, new_addr, new_pud, pud);
+	flush_pud_tlb_range(vma, old_addr, old_addr + HPAGE_PUD_SIZE);
+	if (new_ptl != old_ptl)
+		spin_unlock(new_ptl);
+	spin_unlock(old_ptl);
+
+	return true;
+}
+
 enum pgt_entry {
 	NORMAL_PMD,
 	HPAGE_PMD,
 	NORMAL_PUD,
+	HPAGE_PUD,
 };
 
 /*
@@ -347,6 +388,7 @@ static __always_inline unsigned long get_extent(enum pgt_entry entry,
 		mask = PMD_MASK;
 		size = PMD_SIZE;
 		break;
+	case HPAGE_PUD:
 	case NORMAL_PUD:
 		mask = PUD_MASK;
 		size = PUD_SIZE;
@@ -395,6 +437,12 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
 			move_huge_pmd(vma, old_addr, new_addr, old_entry,
 				      new_entry);
 		break;
+	case HPAGE_PUD:
+		moved = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PUD) &&
+			move_huge_pud(vma, old_addr, new_addr, old_entry,
+				      new_entry);
+		break;
+
 	default:
 		WARN_ON_ONCE(1);
 		break;
@@ -429,15 +477,23 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 		 * PUD level if possible.
 		 */
 		extent = get_extent(NORMAL_PUD, old_addr, old_end, new_addr);
-		if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
-			pud_t *old_pud, *new_pud;
 
-			old_pud = get_old_pud(vma->vm_mm, old_addr);
-			if (!old_pud)
+		old_pud = get_old_pud(vma->vm_mm, old_addr);
+		if (!old_pud)
+			continue;
+		new_pud = alloc_new_pud(vma->vm_mm, vma, new_addr);
+		if (!new_pud)
+			break;
+		if (pud_trans_huge(*old_pud) || pud_devmap(*old_pud)) {
+			if (extent == HPAGE_PUD_SIZE) {
+				move_pgt_entry(HPAGE_PUD, vma, old_addr, new_addr,
+					       old_pud, new_pud, need_rmap_locks);
+				/* We ignore and continue on error? */
 				continue;
-			new_pud = alloc_new_pud(vma->vm_mm, vma, new_addr);
-			if (!new_pud)
-				break;
+			}
+		} else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
+			pud_t *old_pud, *new_pud;
+
 			if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr,
 					   old_pud, new_pud, need_rmap_locks))
 				continue;


>
> Are huge pud thp only allowed with DAX vmas?
>
>
> -aneesh