Re: [PATCH v5 17/18] btrfs: disable defrag on pre-content watched files

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Sep 4, 2024 at 10:29 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> We queue up inodes to be defrag'ed asynchronously, which means we do not
> have their original file for readahead.  This means that the code to
> skip readahead on pre-content watched files will not run, and we could
> potentially read in empty pages.
>
> Handle this corner case by disabling defrag on files that are currently
> being watched for pre-content events.

IIUC, you are disabling the *start* of async defrag.
What happens if defrag has already started and then a pre-content
watch is set up?

Do we care about this corner of a corner case?

Thanks,
Amir.

>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/ioctl.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index e0a664b8a46a..529f7416814f 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -2640,6 +2640,15 @@ static int btrfs_ioctl_defrag(struct file *file, void __user *argp)
>                         goto out;
>                 }
>
> +               /*
> +                * Don't allow defrag on pre-content watched files, as it could
> +                * populate the page cache with 0's via readahead.
> +                */
> +               if (fsnotify_file_has_pre_content_watches(file)) {
> +                       ret = -EINVAL;
> +                       goto out;
> +               }
> +
>                 if (argp) {
>                         if (copy_from_user(&range, argp, sizeof(range))) {
>                                 ret = -EFAULT;
> --
> 2.43.0
>