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Date:   Sat, 19 Dec 2020 11:48:40 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] inotify, memcg: account inotify instances to kmemcg

On Sat, Dec 19, 2020 at 12:11 AM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> Currently the fs sysctl inotify/max_user_instances is used to limit the
> number of inotify instances on the system. For systems running multiple
> workloads, the per-user namespace sysctl max_inotify_instances can be
> used to further partition inotify instances. However there is no easy
> way to set a sensible system level max limit on inotify instances and
> further partition it between the workloads. It is much easier to charge
> the underlying resource (i.e. memory) behind the inotify instances to
> the memcg of the workload and let their memory limits limit the number
> of inotify instances they can create.

Not that I have a problem with this patch, but what problem does it try to
solve? Are you concerned of users depleting system memory by creating
userns's and allocating 128 * (struct fsnotify_group) at a time?

IMO, that is not what max_user_instances was meant to protect against.
There are two reasons I can think of to limit user instances:
1. Pre-memgc, user allocation of events is limited to
    <max_user_instances>*<max_queued_events>
2. Performance penalty. User can place <max_user_instances>
    watches on the same "hot" directory, that will cause any access to
    that directory by any task on the system to pay the penalty of traversing
    <max_user_instances> marks and attempt to queue <max_user_instances>
    events. That cost, including <max_user_instances> inotify_merge() loops
    could be significant

#1 is not a problem anymore, since you already took care of accounting events
to the user's memcg.
#2 is not addressed by your patch.

>
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> ---
>  fs/notify/group.c                | 14 ++++++++++++--
>  fs/notify/inotify/inotify_user.c |  5 +++--
>  include/linux/fsnotify_backend.h |  2 ++
>  3 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/fs/notify/group.c b/fs/notify/group.c
> index a4a4b1c64d32..fab3cfdb4d9e 100644
> --- a/fs/notify/group.c
> +++ b/fs/notify/group.c
> @@ -114,11 +114,12 @@ EXPORT_SYMBOL_GPL(fsnotify_put_group);
>  /*
>   * Create a new fsnotify_group and hold a reference for the group returned.
>   */
> -struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
> +struct fsnotify_group *fsnotify_alloc_group_gfp(const struct fsnotify_ops *ops,
> +                                               gfp_t gfp)
>  {
>         struct fsnotify_group *group;
>
> -       group = kzalloc(sizeof(struct fsnotify_group), GFP_KERNEL);
> +       group = kzalloc(sizeof(struct fsnotify_group), gfp);
>         if (!group)
>                 return ERR_PTR(-ENOMEM);
>
> @@ -139,6 +140,15 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
>
>         return group;
>  }
> +EXPORT_SYMBOL_GPL(fsnotify_alloc_group_gfp);
> +
> +/*
> + * Create a new fsnotify_group and hold a reference for the group returned.
> + */
> +struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
> +{
> +       return fsnotify_alloc_group_gfp(ops, GFP_KERNEL);
> +}
>  EXPORT_SYMBOL_GPL(fsnotify_alloc_group);
>
>  int fsnotify_fasync(int fd, struct file *file, int on)
> diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
> index 59c177011a0f..7cb528c6154c 100644
> --- a/fs/notify/inotify/inotify_user.c
> +++ b/fs/notify/inotify/inotify_user.c
> @@ -632,11 +632,12 @@ static struct fsnotify_group *inotify_new_group(unsigned int max_events)
>         struct fsnotify_group *group;
>         struct inotify_event_info *oevent;
>
> -       group = fsnotify_alloc_group(&inotify_fsnotify_ops);
> +       group = fsnotify_alloc_group_gfp(&inotify_fsnotify_ops,
> +                                        GFP_KERNEL_ACCOUNT);
>         if (IS_ERR(group))
>                 return group;
>
> -       oevent = kmalloc(sizeof(struct inotify_event_info), GFP_KERNEL);
> +       oevent = kmalloc(sizeof(struct inotify_event_info), GFP_KERNEL_ACCOUNT);
>         if (unlikely(!oevent)) {
>                 fsnotify_destroy_group(group);
>                 return ERR_PTR(-ENOMEM);

Any reason why you did not include fanotify in this patch?

Thanks,
Amir.

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