[PATCH RFC v2 10/12] mm/hugetlb: Use hugetlb walker lock in walk_hugetlb_range()

[Peter Xu <peterx@xxxxxxxxxx>]

Hugetlb walker lock makes sure the pte_t* won't go away from under us.  One
thing to mention is there're two hugetlb_entry() users that can yield the
thread within hugetlb_entry(), that'll need to add unlock/lock pair around
the yield, meanwhile document hugetlb_entry() explaining the lock for
sleepable hugetlb_entry()s.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 arch/s390/mm/gmap.c      | 2 ++
 fs/proc/task_mmu.c       | 2 ++
 include/linux/pagewalk.h | 9 ++++++++-
 mm/pagewalk.c            | 2 ++
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 02d15c8dc92e..fb2938e8d1c7 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2644,7 +2644,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
 	end = start + HPAGE_SIZE - 1;
 	__storage_key_init_range(start, end);
 	set_bit(PG_arch_1, &page->flags);
+	hugetlb_walker_unlock();
 	cond_resched();
+	hugetlb_walker_lock();
 	return 0;
 }
 
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 89338950afd3..ed750a52e60b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1612,7 +1612,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
 			frame++;
 	}
 
+	hugetlb_walker_unlock();
 	cond_resched();
+	hugetlb_walker_lock();
 
 	return err;
 }
diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 959f52e5867d..7fa3724c6eb5 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -21,7 +21,14 @@ struct mm_walk;
  *			depth is -1 if not known, 0:PGD, 1:P4D, 2:PUD, 3:PMD.
  *			Any folded depths (where PTRS_PER_P?D is equal to 1)
  *			are skipped.
- * @hugetlb_entry:	if set, called for each hugetlb entry
+ * @hugetlb_entry:	if set, called for each hugetlb entry.  Note that
+ *			each pte_t* is protected by hugetlb_walker_lock(),
+ *			and the lock does not allow sleep.  If explicit
+ *			sleep in the entry fn needed, the caller needs to
+ *			release the lock (hugetlb_walker_unlock()), then
+ *			relock it (hugetlb_walker_lock()) before return.
+ *			After the unlock, the pte_t* may become invalid
+ *			anytime so cannot be accessed anymore.
  * @test_walk:		caller specific callback function to determine whether
  *			we walk over the current vma or not. Returning 0 means
  *			"do page table walk over the current vma", returning
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 7f1c9b274906..abf310011ab1 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
 	const struct mm_walk_ops *ops = walk->ops;
 	int err = 0;
 
+	hugetlb_walker_lock();
 	do {
 		next = hugetlb_entry_end(h, addr, end);
 		pte = huge_pte_offset(walk->mm, addr & hmask, sz);
@@ -314,6 +315,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
 		if (err)
 			break;
 	} while (addr = next, addr != end);
+	hugetlb_walker_unlock();
 
 	return err;
 }
-- 
2.37.3