Re: [PATCH v6 8/9] vfs: update times after copying data in __generic_file_write_iter

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Sep 30, 2022 at 2:30 PM Jeff Layton <jlayton@kernel.org> wrote:
>
> The c/mtime and i_version currently get updated before the data is
> copied (or a DIO write is issued), which is problematic for NFS.
>
> READ+GETATTR can race with a write (even a local one) in such a way as
> to make the client associate the state of the file with the wrong change
> attribute. That association can persist indefinitely if the file sees no
> further changes.
>
> Move the setting of times to the bottom of the function in
> __generic_file_write_iter and only update it if something was
> successfully written.
>

This solution is wrong for several reasons:

1. There is still file_update_time() in ->page_mkwrite() so you haven't
    solved the problem completely
2. The other side of the coin is that post crash state is more likely to end
    up data changes without mtime/ctime change

If I read the problem description correctly, then a solution that invalidates
the NFS cache before AND after the write would be acceptable. Right?
Would an extra i_version bump after the write solve the race?

> If the time update fails, log a warning once, but don't fail the write.
> All of the existing callers use update_time functions that don't fail,
> so we should never trip this.
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  mm/filemap.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 15800334147b..72c0ceb75176 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3812,10 +3812,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>         if (err)
>                 goto out;
>
> -       err = file_update_time(file);
> -       if (err)
> -               goto out;
> -
>         if (iocb->ki_flags & IOCB_DIRECT) {
>                 loff_t pos, endbyte;
>
> @@ -3868,6 +3864,19 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>                         iocb->ki_pos += written;
>         }
>  out:
> +       if (written > 0) {
> +               err = file_update_time(file);
> +               /*
> +                * There isn't much we can do at this point if updating the
> +                * times fails after a successful write. The times and i_version
> +                * should still be updated in the inode, and it should still be
> +                * marked dirty, so hopefully the next inode update will catch it.
> +                * Log a warning once so we have a record that something untoward
> +                * has occurred.
> +                */
> +               WARN_ONCE(err, "Failed to update m/ctime after write: %ld\n", err);

pr_warn_once() please - this is not a programming assertion.

Thanks,
Amir.