[PATCH v2 08/47] mm/filemap: finish fault cleanup before dropping the fault lock

["David Hildenbrand (Arm)" <david@kernel.org>]

On a retried fault, a fatal signal interrupting the folio lock wait
makes lock_folio_maybe_drop_mmap() drop the fault lock without pinning
the file. However, filemap_fault() may still need to release
mapping->invalidate_lock. A concurrent munmap() can release the last
file reference and free the mapping before that unlock.

Defer dropping the fault lock to filemap_fault()'s retry cleanup, after
releasing invalidate_lock. If the fault lock was already dropped for
I/O, release the existing file reference instead. Keep the fault lock
held for FAULT_FLAG_RETRY_NOWAIT as before.

Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock")
Cc: stable@vger.kernel.org
Reported-by: Chris Mason <mason@kernel.org>
Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/filemap.c | 37 +++++++++++++------------------------
 1 file changed, 13 insertions(+), 24 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6b5e1d866a9e..b04be64a5069 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3308,11 +3308,11 @@ loff_t mapping_seek_hole_data(struct address_space *mapping, loff_t start,
  * @folio - the folio to lock.
  * @fpin - the pointer to the file we may pin (or is already pinned).
  *
- * This works similar to lock_folio_or_retry in that it can drop the
- * mmap_lock.  It differs in that it actually returns the folio locked
- * if it returns 1 and 0 if it couldn't lock the folio.  If we did have
- * to drop the mmap_lock then fpin will point to the pinned file and
- * needs to be fput()'ed at a later point.
+ * Returns 1 with the folio locked, or 0 if locking failed.
+ *
+ * On both entry and return, a non-NULL *fpin means the fault lock has
+ * already been dropped and the caller must eventually fput() the file.
+ * Otherwise the fault lock is still held, including on failure.
  */
 static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio,
 				     struct file **fpin)
@@ -3329,21 +3329,9 @@ static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio,
 		return 0;
 
 	*fpin = maybe_unlock_mmap_for_io(vmf, *fpin);
-	if (vmf->flags & FAULT_FLAG_KILLABLE) {
-		if (__folio_lock_killable(folio)) {
-			/*
-			 * We didn't have the right flags to drop the
-			 * fault lock, but all fault_handlers only check
-			 * for fatal signals if we return VM_FAULT_RETRY,
-			 * so we need to drop the fault lock here and
-			 * return 0 if we don't have a fpin.
-			 */
-			if (*fpin == NULL)
-				release_fault_lock(vmf);
-			return 0;
-		}
-	} else
-		__folio_lock(folio);
+	if (vmf->flags & FAULT_FLAG_KILLABLE)
+		return !__folio_lock_killable(folio);
+	__folio_lock(folio);
 
 	return 1;
 }
@@ -3572,7 +3560,7 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
  * vma->vm_mm->mmap_lock must be held on entry.
  *
  * If our return value has VM_FAULT_RETRY set, it's because the mmap_lock
- * may be dropped before doing I/O or by lock_folio_maybe_drop_mmap().
+ * may be dropped before doing I/O or when aborting an interrupted wait.
  *
  * If our return value does not have VM_FAULT_RETRY set, the mmap_lock
  * has not been released.
@@ -3728,9 +3716,8 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 
 out_retry:
 	/*
-	 * We dropped the mmap_lock, we need to return to the fault handler to
-	 * re-find the vma and come back and find our hopefully still populated
-	 * page.
+	 * With fpin set, the fault lock was already dropped. Otherwise it is
+	 * still held. Release invalidate_lock before dropping either protection.
 	 */
 	if (!IS_ERR(folio))
 		folio_put(folio);
@@ -3738,6 +3725,8 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 		filemap_invalidate_unlock_shared(mapping);
 	if (fpin)
 		fput(fpin);
+	else if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
+		release_fault_lock(vmf);
 	return ret | VM_FAULT_RETRY;
 }
 EXPORT_SYMBOL(filemap_fault);

-- 
2.43.0