Re: [PATCH v2 09/10] mm/hugetlb: Introduce hugetlb_walk()

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On 12/7/22 12:31, Peter Xu wrote:
huge_pte_offset() is the main walker function for hugetlb pgtables.  The
name is not really representing what it does, though.

Instead of renaming it, introduce a wrapper function called hugetlb_walk()
which will use huge_pte_offset() inside.  Assert on the locks when walking
the pgtable.

Note, the vma lock assertion will be a no-op for private mappings.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
  fs/hugetlbfs/inode.c    |  4 +---
  fs/userfaultfd.c        |  6 ++----
  include/linux/hugetlb.h | 39 +++++++++++++++++++++++++++++++++++++++
  mm/hugetlb.c            | 32 +++++++++++++-------------------
  mm/page_vma_mapped.c    |  2 +-
  mm/pagewalk.c           |  4 +---
  6 files changed, 57 insertions(+), 30 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index fdb16246f46e..48f1a8ad2243 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -388,9 +388,7 @@ static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
  {
  	pte_t *ptep, pte;
- ptep = huge_pte_offset(vma->vm_mm, addr,
-			huge_page_size(hstate_vma(vma)));
-
+	ptep = hugetlb_walk(vma, addr, huge_page_size(hstate_vma(vma)));
  	if (!ptep)
  		return false;
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index a602f008dde5..f31fe1a9f4c5 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -237,14 +237,12 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
  					 unsigned long flags,
  					 unsigned long reason)
  {
-	struct mm_struct *mm = ctx->mm;
  	pte_t *ptep, pte;
  	bool ret = true;
- mmap_assert_locked(mm);
-
-	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
+	mmap_assert_locked(ctx->mm);
+ ptep = hugetlb_walk(vma, address, vma_mmu_pagesize(vma));
  	if (!ptep)
  		goto out;
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 81efd9b9baa2..1c20cbbf3d22 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -2,6 +2,7 @@
  #ifndef _LINUX_HUGETLB_H
  #define _LINUX_HUGETLB_H
+#include <linux/mm.h>
  #include <linux/mm_types.h>
  #include <linux/mmdebug.h>
  #include <linux/fs.h>
@@ -196,6 +197,11 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
   * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE.
   * Returns the pte_t* if found, or NULL if the address is not mapped.
   *
+ * IMPORTANT: we should normally not directly call this function, instead
+ * this is only a common interface to implement arch-specific walker.
+ * Please consider using the hugetlb_walk() helper to make sure of the
+ * correct locking is satisfied.

Or:

"Please use hugetlb_walk() instead, because that will attempt to verify
the locking for you."

+ *
   * Since this function will walk all the pgtable pages (including not only
   * high-level pgtable page, but also PUD entry that can be unshared
   * concurrently for VM_SHARED), the caller of this function should be
@@ -1229,4 +1235,37 @@ bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr);
  #define flush_hugetlb_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
  #endif
+static inline bool
+__vma_shareable_flags_pmd(struct vm_area_struct *vma)
+{
+	return vma->vm_flags & (VM_MAYSHARE | VM_SHARED) &&
+		vma->vm_private_data;
+}
+
+/*
+ * Safe version of huge_pte_offset() to check the locks.  See comments
+ * above huge_pte_offset().
+ */

It is odd to say that functionA() is a safe version of functionB(), if the
names are completely different.

At this point, it is very clear that huge_pte_offset() should be renamed.
I'd suggest something like one of these:

    __hugetlb_walk()
    hugetlb_walk_raw()

+static inline pte_t *
+hugetlb_walk(struct vm_area_struct *vma, unsigned long addr, unsigned long sz)
+{
+#if defined(CONFIG_HUGETLB_PAGE) && \
+	defined(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) && defined(CONFIG_LOCKDEP)
+	struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
+
+	/*
+	 * If pmd sharing possible, locking needed to safely walk the
+	 * hugetlb pgtables.  More information can be found at the comment
+	 * above huge_pte_offset() in the same file.
+	 *
+	 * NOTE: lockdep_is_held() is only defined with CONFIG_LOCKDEP.
+	 */
+	if (__vma_shareable_flags_pmd(vma))
+		WARN_ON_ONCE(!lockdep_is_held(&vma_lock->rw_sema) &&
+			     !lockdep_is_held(
+				 &vma->vm_file->f_mapping->i_mmap_rwsem));
+#endif
+	return huge_pte_offset(vma->vm_mm, addr, sz);
+}

Let's please not slice up C functions with ifdefs. Instead, stick to the
standard approach of

#ifdef X
functionC()
{
	...implementation
}
#else
functionC()
{
	...simpler or shorter or stub implementation
}

+
  #endif /* _LINUX_HUGETLB_H */
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f42399522805..e3500c087893 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4814,7 +4814,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
  	} else {
  		/*
  		 * For shared mappings the vma lock must be held before
-		 * calling huge_pte_offset in the src vma. Otherwise, the
+		 * calling hugetlb_walk() in the src vma. Otherwise, the
  		 * returned ptep could go away if part of a shared pmd and
  		 * another thread calls huge_pmd_unshare.
  		 */
@@ -4824,7 +4824,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
  	last_addr_mask = hugetlb_mask_last_page(h);
  	for (addr = src_vma->vm_start; addr < src_vma->vm_end; addr += sz) {
  		spinlock_t *src_ptl, *dst_ptl;
-		src_pte = huge_pte_offset(src, addr, sz);
+		src_pte = hugetlb_walk(src_vma, addr, sz);
  		if (!src_pte) {
  			addr |= last_addr_mask;
  			continue;
@@ -5028,7 +5028,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
  	hugetlb_vma_lock_write(vma);
  	i_mmap_lock_write(mapping);
  	for (; old_addr < old_end; old_addr += sz, new_addr += sz) {
-		src_pte = huge_pte_offset(mm, old_addr, sz);
+		src_pte = hugetlb_walk(vma, old_addr, sz);
  		if (!src_pte) {
  			old_addr |= last_addr_mask;
  			new_addr |= last_addr_mask;
@@ -5091,7 +5091,7 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
  	last_addr_mask = hugetlb_mask_last_page(h);
  	address = start;
  	for (; address < end; address += sz) {
-		ptep = huge_pte_offset(mm, address, sz);
+		ptep = hugetlb_walk(vma, address, sz);
  		if (!ptep) {
  			address |= last_addr_mask;
  			continue;
@@ -5404,7 +5404,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
  			mutex_lock(&hugetlb_fault_mutex_table[hash]);
  			hugetlb_vma_lock_read(vma);
  			spin_lock(ptl);
-			ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
+			ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
  			if (likely(ptep &&
  				   pte_same(huge_ptep_get(ptep), pte)))
  				goto retry_avoidcopy;
@@ -5442,7 +5442,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
  	 * before the page tables are altered
  	 */
  	spin_lock(ptl);
-	ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
+	ptep = hugetlb_walk(vma, haddr, huge_page_size(h));
  	if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
  		/* Break COW or unshare */
  		huge_ptep_clear_flush(vma, haddr, ptep);
@@ -6227,7 +6227,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
  		return NULL;
hugetlb_vma_lock_read(vma);
-	pte = huge_pte_offset(mm, haddr, huge_page_size(h));
+	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
  	if (!pte)
  		goto out_unlock;
@@ -6292,8 +6292,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
  		 *
  		 * Note that page table lock is not held when pte is null.
  		 */
-		pte = huge_pte_offset(mm, vaddr & huge_page_mask(h),
-				      huge_page_size(h));
+		pte = hugetlb_walk(vma, vaddr & huge_page_mask(h),
+				   huge_page_size(h));
  		if (pte)
  			ptl = huge_pte_lock(h, mm, pte);
  		absent = !pte || huge_pte_none(huge_ptep_get(pte));
@@ -6479,7 +6479,7 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
  	last_addr_mask = hugetlb_mask_last_page(h);
  	for (; address < end; address += psize) {
  		spinlock_t *ptl;
-		ptep = huge_pte_offset(mm, address, psize);
+		ptep = hugetlb_walk(vma, address, psize);
  		if (!ptep) {
  			address |= last_addr_mask;
  			continue;
@@ -6857,12 +6857,6 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
  		*end = ALIGN(*end, PUD_SIZE);
  }
-static bool __vma_shareable_flags_pmd(struct vm_area_struct *vma)
-{
-	return vma->vm_flags & (VM_MAYSHARE | VM_SHARED) &&
-		vma->vm_private_data;
-}
-
  void hugetlb_vma_lock_read(struct vm_area_struct *vma)
  {
  	if (__vma_shareable_flags_pmd(vma)) {
@@ -7028,8 +7022,8 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
saddr = page_table_shareable(svma, vma, addr, idx);
  		if (saddr) {
-			spte = huge_pte_offset(svma->vm_mm, saddr,
-					       vma_mmu_pagesize(svma));
+			spte = hugetlb_walk(svma, saddr,
+					    vma_mmu_pagesize(svma));
  			if (spte) {
  				get_page(virt_to_page(spte));
  				break;
@@ -7387,7 +7381,7 @@ void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
  	hugetlb_vma_lock_write(vma);
  	i_mmap_lock_write(vma->vm_file->f_mapping);
  	for (address = start; address < end; address += PUD_SIZE) {
-		ptep = huge_pte_offset(mm, address, sz);
+		ptep = hugetlb_walk(vma, address, sz);
  		if (!ptep)
  			continue;
  		ptl = huge_pte_lock(h, mm, ptep);
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 93e13fc17d3c..e97b2e23bd28 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -170,7 +170,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
  			return not_found(pvmw);
/* when pud is not present, pte will be NULL */
-		pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
+		pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
  		if (!pvmw->pte)
  			return false;
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index d98564a7be57..cb23f8a15c13 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -305,13 +305,11 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
  	hugetlb_vma_lock_read(vma);
  	do {
  		next = hugetlb_entry_end(h, addr, end);
-		pte = huge_pte_offset(walk->mm, addr & hmask, sz);
-
+		pte = hugetlb_walk(vma, addr & hmask, sz);
  		if (pte)
  			err = ops->hugetlb_entry(pte, hmask, addr, next, walk);
  		else if (ops->pte_hole)
  			err = ops->pte_hole(addr, next, -1, walk);
-
  		if (err)
  			break;
  	} while (addr = next, addr != end);






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