[PATCH v3 9/9] mm/hugetlb: Introduce hugetlb_walk()

From: Peter Xu
Date: Fri Dec 09 2022 - 12:03:35 EST

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.

Document the last special case in the page_vma_mapped_walk() path where we
don't need any more lock to call hugetlb_walk().

Taking vma lock there is not needed because either: (1) potential callers
of hugetlb pvmw holds i_mmap_rwsem already (from one rmap_walk()), or (2)
the caller will not walk a hugetlb vma at all so the hugetlb code path not
reachable (e.g. in ksm or uprobe paths).

It's slightly implicit for future page_vma_mapped_walk() callers on that
lock requirement. But anyway, when one day this rule breaks, one will get
a straightforward warning in hugetlb_walk() with lockdep, then there'll be
a way out.

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
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 | 9 ++++++---
mm/pagewalk.c | 4 +---
6 files changed, 62 insertions(+), 32 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 969f4be967c6..6a278941ec84 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 d755e2a7c0db..a5e87ec7fa6e 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 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().
+ */
+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);
+}
+
#endif /* _LINUX_HUGETLB_H */
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9d8bb6508288..b20120d14a71 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);
@@ -6228,7 +6228,7 @@ 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;

@@ -6293,8 +6293,8 @@ 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));
@@ -6480,7 +6480,7 @@ 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;
@@ -6858,12 +6858,6 @@ 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)) {
@@ -7029,8 +7023,8 @@ 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;
@@ -7388,7 +7382,7 @@ 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..f3729b23dd0e 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -168,9 +168,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
/* The only possible mapping was handled on last iteration */
if (pvmw->pte)
return not_found(pvmw);
-
- /* when pud is not present, pte will be NULL */
- pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
+ /*
+ * All callers that get here will already hold the
+ * i_mmap_rwsem. Therefore, no additional locks need to be
+ * taken before calling hugetlb_walk().
+ */
+ 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);
--
2.37.3