[PATCH v3 5/8] mm: make folio_lock_fault indicate the state of mmap_lock upon return

[Suren Baghdasaryan <surenb@google.com>]

folio_lock_fault might drop mmap_lock before returning and to extend it
to work with per-VMA locks, the callers will need to know whether the
lock was dropped or is still held. Introduce new fault_flag to indicate
whether the lock got dropped and store it inside vm_fault flags.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm_types.h | 1 +
 mm/filemap.c             | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 79765e3dd8f3..6f0dbef7aa1f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1169,6 +1169,7 @@ enum fault_flag {
 	FAULT_FLAG_UNSHARE =		1 << 10,
 	FAULT_FLAG_ORIG_PTE_VALID =	1 << 11,
 	FAULT_FLAG_VMA_LOCK =		1 << 12,
+	FAULT_FLAG_LOCK_DROPPED =	1 << 13,
 };
 
 typedef unsigned int __bitwise zap_flags_t;
diff --git a/mm/filemap.c b/mm/filemap.c
index 87b335a93530..8ad06d69895b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1723,6 +1723,7 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
 			return VM_FAULT_RETRY;
 
 		mmap_read_unlock(mm);
+		vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
 		if (vmf->flags & FAULT_FLAG_KILLABLE)
 			folio_wait_locked_killable(folio);
 		else
@@ -1735,6 +1736,7 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
 		ret = __folio_lock_killable(folio);
 		if (ret) {
 			mmap_read_unlock(mm);
+			vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
 			return VM_FAULT_RETRY;
 		}
 	} else {
-- 
2.41.0.178.g377b9f9a00-goog