linux-kernel - Re: [PATCH 1/2] fuse: add ioctl to cleanup all backing files

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Message-ID: <CAOQ4uxg13jAJyG8b3CpjKE8FXn3ce=yUCzw+Qc=k29si=FtXaQ@mail.gmail.com>
Date: Fri, 16 Jan 2026 16:39:51 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Chunsheng Luo <luochunsheng@...c.edu>
Cc: miklos@...redi.hu, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] fuse: add ioctl to cleanup all backing files

On Fri, Jan 16, 2026 at 3:28 PM Chunsheng Luo <luochunsheng@...c.edu> wrote:
>
> To simplify crash recovery and reduce performance impact, backing_ids
> are not persisted across daemon restarts. After crash recovery, this
> may lead to resource leaks if backing file resources are not properly
> cleaned up.
>
> Add FUSE_DEV_IOC_BACKING_CLOSE_ALL ioctl to release all backing_ids
> and put backing files. When the FUSE daemon restarts, it can use this
> ioctl to cleanup all backing file resources.
>
> Signed-off-by: Chunsheng Luo <luochunsheng@...c.edu>
> ---
>  fs/fuse/backing.c         | 19 +++++++++++++++++++
>  fs/fuse/dev.c             | 16 ++++++++++++++++
>  fs/fuse/fuse_i.h          |  1 +
>  include/uapi/linux/fuse.h |  1 +
>  4 files changed, 37 insertions(+)
>
> diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
> index 4afda419dd14..e93d797a2cde 100644
> --- a/fs/fuse/backing.c
> +++ b/fs/fuse/backing.c
> @@ -166,6 +166,25 @@ int fuse_backing_close(struct fuse_conn *fc, int backing_id)
>         return err;
>  }
>
> +static int fuse_backing_close_one(int id, void *p, void *data)
> +{
> +       struct fuse_conn *fc = data;
> +
> +       fuse_backing_close(fc, id);
> +
> +       return 0;
> +}
> +
> +int fuse_backing_close_all(struct fuse_conn *fc)
> +{
> +       if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
> +               return -EPERM;
> +
> +       idr_for_each(&fc->backing_files_map, fuse_backing_close_one, fc);
> +
> +       return 0;
> +}
> +

This is not safe and not efficient.
For safety from racing with _open/_close, iteration needs at least
rcu_read_lock(),
but I think it will be much more efficient to zap the entire map with
fuse_backing_files_free()/fuse_backing_files_init().

This of course needs to be synchronized with concurrent _open/_close/_lookup.
This could be done by making c->backing_files_map a struct idr __rcu *
and replace the old and new backing_files_map under spin_lock(&fc->lock);

Then you can call fuse_backing_files_free() on the old backing_files_map
without a lock.

As a side note, fuse_backing_files_free() iteration looks like it may need
cond_resched() if there are a LOT of backing ids, but I am not sure and
this is orthogonal to your change.

Thanks,
Amir.