Re: [PATCH v6 15/21] fanotify: Preallocate per superblock mark error event

Error reporting needs to be done in an atomic context.  This patch
introduces a single error slot for superblock marks that report the
FAN_FS_ERROR event, to be used during event submission.

Signed-off-by: Gabriel Krisman Bertazi <redacted>

---
Changes v5:
  - Restore mark references. (jan)
  - Tie fee slot to the mark lifetime.(jan)
  - Don't reallocate event(jan)
---
 fs/notify/fanotify/fanotify.c      | 12 ++++++++++++
 fs/notify/fanotify/fanotify.h      | 13 +++++++++++++
 fs/notify/fanotify/fanotify_user.c | 31 ++++++++++++++++++++++++++++--
 3 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index ebb6c557cea1..3bf6fd85c634 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c

@@ -855,6 +855,14 @@ static void fanotify_free_name_event(struct fanotify_event *event)
        kfree(FANOTIFY_NE(event));
 }

+static void fanotify_free_error_event(struct fanotify_event *event)
+{
+       /*
+        * The actual event is tied to a mark, and is released on mark
+        * removal
+        */
+}
+
 static void fanotify_free_event(struct fsnotify_event *fsn_event)
 {
        struct fanotify_event *event;

@@ -877,6 +885,9 @@ static void fanotify_free_event(struct fsnotify_event *fsn_event)
        case FANOTIFY_EVENT_TYPE_OVERFLOW:
                kfree(event);
                break;
+       case FANOTIFY_EVENT_TYPE_FS_ERROR:
+               fanotify_free_error_event(event);
+               break;
        default:
                WARN_ON_ONCE(1);
        }

@@ -894,6 +905,7 @@ static void fanotify_free_mark(struct fsnotify_mark *mark)
        if (mark->flags & FANOTIFY_MARK_FLAG_SB_MARK) {
                struct fanotify_sb_mark *fa_mark = FANOTIFY_SB_MARK(mark);

+               kfree(fa_mark->fee_slot);
                kmem_cache_free(fanotify_sb_mark_cache, fa_mark);
        } else {
                kmem_cache_free(fanotify_mark_cache, mark);

diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index b3ab620822c2..3f03333df32f 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h

@@ -139,6 +139,7 @@ enum fanotify_mark_bits {

 struct fanotify_sb_mark {
        struct fsnotify_mark fsn_mark;
+       struct fanotify_error_event *fee_slot;
 };

 static inline

@@ -161,6 +162,7 @@ enum fanotify_event_type {
        FANOTIFY_EVENT_TYPE_PATH,
        FANOTIFY_EVENT_TYPE_PATH_PERM,
        FANOTIFY_EVENT_TYPE_OVERFLOW, /* struct fanotify_event */
+       FANOTIFY_EVENT_TYPE_FS_ERROR, /* struct fanotify_error_event */
        __FANOTIFY_EVENT_TYPE_NUM
 };

@@ -216,6 +218,17 @@ FANOTIFY_NE(struct fanotify_event *event)
        return container_of(event, struct fanotify_name_event, fae);
 }

+struct fanotify_error_event {
+       struct fanotify_event fae;
+       struct fanotify_sb_mark *sb_mark; /* Back reference to the mark. */
+};
+
+static inline struct fanotify_error_event *
+FANOTIFY_EE(struct fanotify_event *event)
+{
+       return container_of(event, struct fanotify_error_event, fae);
+}
+
 static inline __kernel_fsid_t *fanotify_event_fsid(struct fanotify_event *event)
 {
        if (event->type == FANOTIFY_EVENT_TYPE_FID)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 54107f1533d5..b77030386d7f 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c

@@ -947,8 +947,10 @@ static struct fsnotify_mark *fanotify_alloc_mark(struct fsnotify_group *group,

        fsnotify_init_mark(mark, group);

-       if (type == FSNOTIFY_OBJ_TYPE_SB)
+       if (type == FSNOTIFY_OBJ_TYPE_SB) {
                mark->flags |= FANOTIFY_MARK_FLAG_SB_MARK;
+               sb_mark->fee_slot = NULL;
+       }

        return mark;
 }

@@ -999,6 +1001,7 @@ static int fanotify_add_mark(struct fsnotify_group *group,
 {
        struct fsnotify_mark *fsn_mark;
        __u32 added;
+       int ret = 0;

        mutex_lock(&group->mark_mutex);
        fsn_mark = fsnotify_find_mark(connp, group);

@@ -1009,13 +1012,37 @@ static int fanotify_add_mark(struct fsnotify_group *group,
                        return PTR_ERR(fsn_mark);
                }
        }
+
+       /*
+        * Error events are allocated per super-block mark only if
+        * strictly needed (i.e. FAN_FS_ERROR was requested).
+        */
+       if (type == FSNOTIFY_OBJ_TYPE_SB && !(flags & FAN_MARK_IGNORED_MASK) &&
+           (mask & FAN_FS_ERROR)) {
+               struct fanotify_sb_mark *sb_mark = FANOTIFY_SB_MARK(fsn_mark);
+
+               if (!sb_mark->fee_slot) {
+                       struct fanotify_error_event *fee =
+                               kzalloc(sizeof(*fee), GFP_KERNEL_ACCOUNT);
+                       if (!fee) {
+                               ret = -ENOMEM;
+                               goto out;
+                       }
+                       fanotify_init_event(&fee->fae, 0, FS_ERROR);
+                       fee->sb_mark = sb_mark;