From patchwork series 420274

https://patchwork.ozlabs.org//project/linuxppc-dev/list/?series=420274

Author: Fox Snowpatch

Documentation/mm/split_page_table_lock.rst

@@ -16,9 +16,13 @@ There are helpers to lock/unlock a table and other accessor functions:
  - pte_offset_map_lock()
 	maps PTE and takes PTE table lock, returns pointer to PTE with
 	pointer to its PTE table lock, or returns NULL if no PTE table;
- - pte_offset_map_nolock()
+ - pte_offset_map_readonly_nolock()
 	maps PTE, returns pointer to PTE with pointer to its PTE table
 	lock (not taken), or returns NULL if no PTE table;
+ - pte_offset_map_maywrite_nolock()
+	maps PTE, returns pointer to PTE with pointer to its PTE table
+	lock (not taken) and the value of its pmd entry, or returns NULL
+	if no PTE table;
  - pte_offset_map()
 	maps PTE, returns pointer to PTE, or returns NULL if no PTE table;
  - pte_unmap()

arch/arm/mm/fault-armv.c

@@ -94,6 +94,7 @@ static int adjust_pte(struct vm_area_struct *vma, unsigned long address,
 	pud_t *pud;
 	pmd_t *pmd;
 	pte_t *pte;
+	pmd_t pmdval;
 	int ret;
 
 	pgd = pgd_offset(vma->vm_mm, address);
@@ -112,16 +113,22 @@ static int adjust_pte(struct vm_area_struct *vma, unsigned long address,
 	if (pmd_none_or_clear_bad(pmd))
 		return 0;
 
+again:
 	/*
 	 * This is called while another page table is mapped, so we
 	 * must use the nested version.  This also means we need to
 	 * open-code the spin-locking.
 	 */
-	pte = pte_offset_map_nolock(vma->vm_mm, pmd, address, &ptl);
+	pte = pte_offset_map_maywrite_nolock(vma->vm_mm, pmd, address, &pmdval, &ptl);
 	if (!pte)
 		return 0;
 
 	do_pte_lock(ptl);
+	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
+		do_pte_unlock(ptl);
+		pte_unmap(pte);
+		goto again;
+	}
 
 	ret = do_adjust_pte(vma, address, pfn, pte);
 

arch/powerpc/mm/pgtable.c

@@ -392,7 +392,7 @@ void assert_pte_locked(struct mm_struct *mm, unsigned long addr)
 	 */
 	if (pmd_none(*pmd))
 		return;
-	pte = pte_offset_map_nolock(mm, pmd, addr, &ptl);
+	pte = pte_offset_map_readonly_nolock(mm, pmd, addr, &ptl);
 	BUG_ON(!pte);
 	assert_spin_locked(ptl);
 	pte_unmap(pte);

include/linux/mm.h

@@ -2986,8 +2986,11 @@ static inline pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
 	return pte;
 }
 
-pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
-			unsigned long addr, spinlock_t **ptlp);
+pte_t *pte_offset_map_readonly_nolock(struct mm_struct *mm, pmd_t *pmd,
+				      unsigned long addr, spinlock_t **ptlp);
+pte_t *pte_offset_map_maywrite_nolock(struct mm_struct *mm, pmd_t *pmd,
+				      unsigned long addr, pmd_t *pmdvalp,
+				      spinlock_t **ptlp);
 
 #define pte_unmap_unlock(pte, ptl)	do {		\
 	spin_unlock(ptl);				\

mm/filemap.c

@@ -3231,8 +3231,8 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
 	if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID))
 		return 0;
 
-	ptep = pte_offset_map_nolock(vma->vm_mm, vmf->pmd, vmf->address,
-				     &vmf->ptl);
+	ptep = pte_offset_map_readonly_nolock(vma->vm_mm, vmf->pmd,
+					      vmf->address, &vmf->ptl);
 	if (unlikely(!ptep))
 		return VM_FAULT_NOPAGE;
 

mm/khugepaged.c

@@ -1009,7 +1009,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
 		};
 
 		if (!pte++) {
-			pte = pte_offset_map_nolock(mm, pmd, address, &ptl);
+			/*
+			 * Here the ptl is only used to check pte_same() in
+			 * do_swap_page(), so readonly version is enough.
+			 */
+			pte = pte_offset_map_readonly_nolock(mm, pmd, address, &ptl);
 			if (!pte) {
 				mmap_read_unlock(mm);
 				result = SCAN_PMD_NULL;
@@ -1598,14 +1602,17 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 	if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
 		pml = pmd_lock(mm, pmd);
 
-	start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
+	start_pte = pte_offset_map_maywrite_nolock(mm, pmd, haddr, &pgt_pmd, &ptl);
 	if (!start_pte)		/* mmap_lock + page lock should prevent this */
 		goto abort;
 	if (!pml)
 		spin_lock(ptl);
 	else if (ptl != pml)
 		spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
 
+	if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd))))
+		goto abort;
+
 	/* step 2: clear page table and adjust rmap */
 	for (i = 0, addr = haddr, pte = start_pte;
 	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
@@ -1651,6 +1658,16 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 	/* step 4: remove empty page table */
 	if (!pml) {
 		pml = pmd_lock(mm, pmd);
+		/*
+		 * We called pte_unmap() and release the ptl before acquiring
+		 * the pml, which means we left the RCU critical section, so the
+		 * PTE page may have been freed, so we must do pmd_same() check
+		 * before reacquiring the ptl.
+		 */
+		if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
+			spin_unlock(pml);
+			goto pmd_change;
+		}
 		if (ptl != pml)
 			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
 	}
@@ -1682,6 +1699,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 		pte_unmap_unlock(start_pte, ptl);
 	if (pml && pml != ptl)
 		spin_unlock(pml);
+pmd_change:
 	if (notified)
 		mmu_notifier_invalidate_range_end(&range);
 drop_folio:
@@ -1703,6 +1721,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 		spinlock_t *pml;
 		spinlock_t *ptl;
 		bool skipped_uffd = false;
+		pte_t *pte;
 
 		/*
 		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
@@ -1738,11 +1757,25 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 					addr, addr + HPAGE_PMD_SIZE);
 		mmu_notifier_invalidate_range_start(&range);
 
+		pte = pte_offset_map_maywrite_nolock(mm, pmd, addr, &pgt_pmd, &ptl);
+		if (!pte) {
+			mmu_notifier_invalidate_range_end(&range);
+			continue;
+		}
+
 		pml = pmd_lock(mm, pmd);
-		ptl = pte_lockptr(mm, pmd);
 		if (ptl != pml)
 			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
 
+		if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
+			pte_unmap_unlock(pte, ptl);
+			if (ptl != pml)
+				spin_unlock(pml);
+			mmu_notifier_invalidate_range_end(&range);
+			continue;
+		}
+		pte_unmap(pte);
+
 		/*
 		 * Huge page lock is still held, so normally the page table
 		 * must remain empty; and we have already skipped anon_vma

mm/memory.c

@@ -1083,6 +1083,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 	struct mm_struct *src_mm = src_vma->vm_mm;
 	pte_t *orig_src_pte, *orig_dst_pte;
 	pte_t *src_pte, *dst_pte;
+	pmd_t pmdval;
 	pte_t ptent;
 	spinlock_t *src_ptl, *dst_ptl;
 	int progress, max_nr, ret = 0;
@@ -1108,7 +1109,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 		ret = -ENOMEM;
 		goto out;
 	}
-	src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_ptl);
+	src_pte = pte_offset_map_maywrite_nolock(src_mm, src_pmd, addr, NULL,
+						 &src_ptl);
 	if (!src_pte) {
 		pte_unmap_unlock(dst_pte, dst_ptl);
 		/* ret == 0 */
@@ -5504,9 +5506,14 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 * pmd by anon khugepaged, since that takes mmap_lock in write
 		 * mode; but shmem or file collapse to THP could still morph
 		 * it into a huge pmd: just retry later if so.
+		 *
+		 * Use the maywrite version to indicate that vmf->pte will be
+		 * modified, but since we will use pte_same() to detect the
+		 * change of the pte entry, there is no need to get pmdval.
 		 */
-		vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
-						 vmf->address, &vmf->ptl);
+		vmf->pte = pte_offset_map_maywrite_nolock(vmf->vma->vm_mm,
+							  vmf->pmd, vmf->address,
+							  NULL, &vmf->ptl);
 		if (unlikely(!vmf->pte))
 			return 0;
 		vmf->orig_pte = ptep_get_lockless(vmf->pte);

mm/mremap.c

@@ -175,7 +175,12 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 		err = -EAGAIN;
 		goto out;
 	}
-	new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_ptl);
+	/*
+	 * Use the maywrite version to indicate that new_pte will be modified,
+	 * but since we hold the exclusive mmap_lock, there is no need to
+	 * recheck pmd_same() after acquiring the new_ptl.
+	 */
+	new_pte = pte_offset_map_maywrite_nolock(mm, new_pmd, new_addr, NULL, &new_ptl);
 	if (!new_pte) {
 		pte_unmap_unlock(old_pte, old_ptl);
 		err = -EAGAIN;

mm/page_vma_mapped.c

@@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
 	return false;
 }
 
-static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
+static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
+		    spinlock_t **ptlp)
 {
 	pte_t ptent;
+	pmd_t pmdval;
 
 	if (pvmw->flags & PVMW_SYNC) {
 		/* Use the stricter lookup */
@@ -25,17 +27,19 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
 		return !!pvmw->pte;
 	}
 
+again:
 	/*
 	 * It is important to return the ptl corresponding to pte,
 	 * in case *pvmw->pmd changes underneath us; so we need to
 	 * return it even when choosing not to lock, in case caller
 	 * proceeds to loop over next ptes, and finds a match later.
 	 * Though, in most cases, page lock already protects this.
 	 */
-	pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
-					  pvmw->address, ptlp);
+	pvmw->pte = pte_offset_map_maywrite_nolock(pvmw->vma->vm_mm, pvmw->pmd,
+						   pvmw->address, &pmdval, ptlp);
 	if (!pvmw->pte)
 		return false;
+	*pmdvalp = pmdval;
 
 	ptent = ptep_get(pvmw->pte);
 
@@ -69,6 +73,12 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
 	}
 	pvmw->ptl = *ptlp;
 	spin_lock(pvmw->ptl);
+
+	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
+		spin_unlock(pvmw->ptl);
+		goto again;
+	}
+
 	return true;
 }
 
@@ -278,7 +288,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			step_forward(pvmw, PMD_SIZE);
 			continue;
 		}
-		if (!map_pte(pvmw, &ptl)) {
+		if (!map_pte(pvmw, &pmde, &ptl)) {
 			if (!pvmw->pte)
 				goto restart;
 			goto next_pte;
@@ -307,6 +317,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 		if (!pvmw->ptl) {
 			pvmw->ptl = ptl;
 			spin_lock(pvmw->ptl);
+			if (unlikely(!pmd_same(pmde, pmdp_get_lockless(pvmw->pmd)))) {
+				pte_unmap_unlock(pvmw->pte, pvmw->ptl);
+				pvmw->ptl = NULL;
+				pvmw->pte = NULL;
+				goto restart;
+			}
 		}
 		goto this_pte;
 	} while (pvmw->address < end);

mm/pgtable-generic.c

@@ -305,15 +305,30 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
 	return NULL;
 }
 
-pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
-			     unsigned long addr, spinlock_t **ptlp)
+pte_t *pte_offset_map_readonly_nolock(struct mm_struct *mm, pmd_t *pmd,
+				      unsigned long addr, spinlock_t **ptlp)
+{
+	pmd_t pmdval;
+	pte_t *pte;
+
+	pte = __pte_offset_map(pmd, addr, &pmdval);
+	if (likely(pte))
+		*ptlp = pte_lockptr(mm, &pmdval);
+	return pte;
+}
+
+pte_t *pte_offset_map_maywrite_nolock(struct mm_struct *mm, pmd_t *pmd,
+				      unsigned long addr, pmd_t *pmdvalp,
+				      spinlock_t **ptlp)
 {
 	pmd_t pmdval;
 	pte_t *pte;
 
 	pte = __pte_offset_map(pmd, addr, &pmdval);
 	if (likely(pte))
 		*ptlp = pte_lockptr(mm, &pmdval);
+	if (pmdvalp)
+		*pmdvalp = pmdval;
 	return pte;
 }
 
@@ -347,14 +362,21 @@ pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
  * and disconnected table.  Until pte_unmap(pte) unmaps and rcu_read_unlock()s
  * afterwards.
  *
- * pte_offset_map_nolock(mm, pmd, addr, ptlp), above, is like pte_offset_map();
- * but when successful, it also outputs a pointer to the spinlock in ptlp - as
- * pte_offset_map_lock() does, but in this case without locking it.  This helps
- * the caller to avoid a later pte_lockptr(mm, *pmd), which might by that time
- * act on a changed *pmd: pte_offset_map_nolock() provides the correct spinlock
- * pointer for the page table that it returns.  In principle, the caller should
- * recheck *pmd once the lock is taken; in practice, no callsite needs that -
- * either the mmap_lock for write, or pte_same() check on contents, is enough.
+ * pte_offset_map_readonly_nolock(mm, pmd, addr, ptlp), above, is like
+ * pte_offset_map(); but when successful, it also outputs a pointer to the
+ * spinlock in ptlp - as pte_offset_map_lock() does, but in this case without
+ * locking it.  This helps the caller to avoid a later pte_lockptr(mm, *pmd),
+ * which might by that time act on a changed *pmd: pte_offset_map_readonly_nolock()
+ * provides the correct spinlock pointer for the page table that it returns.
+ * For readonly case, the caller does not need to recheck *pmd after the lock is
+ * taken, because the RCU lock will ensure that the PTE page will not be freed.
+ *
+ * pte_offset_map_maywrite_nolock(mm, pmd, addr, pmdvalp, ptlp), above, is like
+ * pte_offset_map_readonly_nolock(); but when successful, it also outputs the
+ * pdmval. For cases where pte or pmd entries may be modified, that is, maywrite
+ * case, this can help the caller recheck *pmd once the lock is taken. In some
+ * cases we can pass NULL to pmdvalp: either the mmap_lock for write, or
+ * pte_same() check on contents, is also enough.
  *
  * Note that free_pgtables(), used after unmapping detached vmas, or when
  * exiting the whole mm, does not take page table lock before freeing a page