[PATCH v2 12/12] mm: change the contract for free_pgtables(), update docs

[Lorenzo Stoakes (ARM)]

Now that page tables are freed after an RCU grace period, it is safe for
read-only page table walkers to walk page table ranges that are being
concurrently torn down, provided the mm is kept alive via mmgrab().

It is however unsafe for writers to do so, as they must obtain an
appropriate lock to do so safely.

Update the pte_offset_map_lock()'s comment block to reflect this.

Similarly update the process addresses documentation.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
---
 Documentation/mm/process_addrs.rst |  6 ++++++
 mm/pgtable-generic.c               | 15 +++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
index a7296f251799..1e65b139f355 100644
--- a/Documentation/mm/process_addrs.rst
+++ b/Documentation/mm/process_addrs.rst
@@ -537,6 +537,12 @@ We establish basic locking rules when interacting with page tables:
 * When changing a page table entry the page table lock for that page table
   **must** be held, except if you can safely assume nobody can access the page
   tables concurrently (such as on invocation of :c:func:`!free_pgtables`).
+* Page tables may be *walked* under RCU alone, as page tables are freed only
+  after an RCU grace period has elapsed. However, any entry found must be
+  revalidated after the page table lock is taken (such as the
+  :c:func:`!pmd_same` recheck performed by :c:func:`!pte_offset_map_lock`)
+  before it is acted upon. Changing an entry always requires the page table
+  lock.
 * Reads from and writes to page table entries must be *appropriately*
   atomic. See the section on atomicity below for details.
 * Populating previously empty entries requires that the mmap or VMA locks are
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index b91b1a98029c..a127e3e8f9b9 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -385,10 +385,17 @@ pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
  * Note: "RO" / "RW" expresses the intended semantics, not that the *kmap* will
  * be read-only/read-write protected.
  *
- * 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
- * table, and may not use RCU at all: "outsiders" like khugepaged should avoid
- * pte_offset_map() and co once the vma is detached from mm or mm_users is zero.
+ * Note that free_pgtables(), used after unmapping detached vmas or when exiting
+ * the whole mm, does not take a page table lock before freeing a page table.
+ *
+ * As page table freeing itself is RCU-safe, page table readers can safely run
+ * concurrently with page table teardown.
+ *
+ * However, writers CANNOT as, without a lock being held, nothing prevents
+ * concurrent teardown.
+ *
+ * Also note that the PGD itself is freed at mmdrop() time, not under RCU - so
+ * the walker must keep the mm alive either by pinning the mm or the VMA.
  */
 pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
 			   unsigned long addr, spinlock_t **ptlp)

-- 
2.55.0