Re: [PATCH v7 18/28] fanotify: Pre-allocate pool of error events

On Fri, Oct 15, 2021 at 9:19 AM Amir Goldstein [off-list ref] wrote:

On Fri, Oct 15, 2021 at 12:39 AM Gabriel Krisman Bertazi
[off-list ref] wrote:

quoted

Error reporting needs to be done in an atomic context.  This patch
introduces a group-wide mempool of error events, shared by all
marks in this group.

Signed-off-by: Gabriel Krisman Bertazi <redacted>
---
 fs/notify/fanotify/fanotify.c      |  3 +++
 fs/notify/fanotify/fanotify.h      | 11 +++++++++++
 fs/notify/fanotify/fanotify_user.c | 26 +++++++++++++++++++++++++-
 include/linux/fsnotify_backend.h   |  2 ++
 4 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 8f152445d75c..01d68dfc74aa 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c

@@ -819,6 +819,9 @@ static void fanotify_free_group_priv(struct fsnotify_group *group)
        if (group->fanotify_data.ucounts)
                dec_ucount(group->fanotify_data.ucounts,
                           UCOUNT_FANOTIFY_GROUPS);
+
+       if (mempool_initialized(&group->fanotify_data.error_events_pool))
+               mempool_exit(&group->fanotify_data.error_events_pool);
 }

 static void fanotify_free_path_event(struct fanotify_event *event)

diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index c42cf8fd7d79..a577e87fac2b 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h

@@ -141,6 +141,7 @@ 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
 };

@@ -196,6 +197,16 @@ FANOTIFY_NE(struct fanotify_event *event)
        return container_of(event, struct fanotify_name_event, fae);
 }

+struct fanotify_error_event {
+       struct fanotify_event fae;
+};
+
+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 66ee3c2805c7..f1cf863d6f9f 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c

@@ -30,6 +30,7 @@
 #define FANOTIFY_DEFAULT_MAX_EVENTS    16384
 #define FANOTIFY_OLD_DEFAULT_MAX_MARKS 8192
 #define FANOTIFY_DEFAULT_MAX_GROUPS    128
+#define FANOTIFY_DEFAULT_FEE_POOL      32

We can probably start with a more generous pool (128?)
It doesn't cost that much.
But anyway, I think this pool needs to auto-grow (up to a maximum size)
instead of having a rigid arbitrary limit.

As long as the pool grows, I don't mind if it start at size 32,
but I just noticed that mempools cannot be accounted to memcg??
Then surely the maximum size need to be kept pretty low.

Thanks,
Amir.