Date:   Tue, 28 Jun 2022 13:09:56 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jiachen Zhang <zhangjiachen.jaycee@...edance.com>,
        Vivek Goyal <vgoyal@...hat.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        xieyongji@...edance.com, fam.zheng@...edance.com,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: Re: [PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)

On Fri, Jun 24, 2022 at 9:03 AM Jiachen Zhang
<zhangjiachen.jaycee@...edance.com> wrote:
>
> Some users may want both the high performance of the writeback_cahe mode and
> a little bit more consistency among FUSE mounts. In the current writeback
> mode implementation, users of one FUSE mount can never see the file
> expansion done by other FUSE mounts.
>
> Based on the suggested writeback V2 patch in the upstream mailing-list [1],
> this commit allows the cmtime and size to be updated from server in
> writeback mode. Compared with the writeback V2 patch in [1], this commit has
> several differences:
>
>     1. Ensure c/mtime are not updated from kernel to server. IOW, the cmtime
>     generated by kernel are just temporary values that are never flushed to
>     server, and they can also be updated by the official server cmtime when
>     the writeback cache is clean.
>
>     2. Skip mtime-based revalidation when fc->auto_inval_data is set with
>     fc->writeback_cache_v2. Because the kernel-generated temporary cmtime
>     are likely not equal to the offical server cmtime.
>
>     3. If any page is ever flushed to the server during FUSE_GETATTR
>     handling on fuse server, even if the cache is clean when
>     fuse_change_attributes() checks, we should not update the i_size. This
>     is because the FUSE_GETATTR may get a staled size before the FUSE_WRITE
>     request changes server inode size. This commit ensures this by
>     increasing attr_version after writeback for writeback_cache_v2. In that
>     case, we should also ensure the ordering of the attr_version updating
>     and the fi->writepages RB-tree updating. So that if a fuse page
>     writeback ever happens during fuse_change_attributes(), either the
>     fi->writepages is not empty, or the attr_version is increased. So we
>     never mistakenly update a stale file size from server to kernel.
>
> With this patch, writeback mode can consider the server c/mtime as the
> official one. When inode attr is timeout or invalidated, kernel has chance
> to see size and c/mtime modified by others.
>
> Together with another patch [2], a FUSE daemon is able to implement
> close-to-open (CTO) consistency like what is done in NFS clients.
>
> [1] https://lore.kernel.org/linux-fsdevel/Ymfu8fGbfYi4FxQ4@miu.piliscsaba.redhat.com
> [2] https://lore.kernel.org/linux-fsdevel/20220608104202.19461-1-zhangjiachen.jaycee@bytedance.com/
>
> Suggested-by: Miklos Szeredi <mszeredi@...hat.com>
> Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@...edance.com>
> ---
>  fs/fuse/file.c            | 17 +++++++++++++++
>  fs/fuse/fuse_i.h          |  3 +++
>  fs/fuse/inode.c           | 44 +++++++++++++++++++++++++++++++++++++--
>  include/uapi/linux/fuse.h |  5 +++++
>  4 files changed, 67 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 9b64e2ff1c96..35bdc7af8468 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1829,6 +1829,15 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
>                  */
>                 fuse_send_writepage(fm, next, inarg->offset + inarg->size);
>         }
> +
> +       if (fc->writeback_cache_v2)
> +               fi->attr_version = atomic64_inc_return(&fc->attr_version);
> +       /*
> +        * Ensure attr_version increases before the page is move out of the
> +        * writepages rb-tree.
> +        */
> +       smp_mb();
> +
>         fi->writectr--;
>         fuse_writepage_finish(fm, wpa);
>         spin_unlock(&fi->lock);
> @@ -1858,10 +1867,18 @@ static struct fuse_file *fuse_write_file_get(struct fuse_inode *fi)
>
>  int fuse_write_inode(struct inode *inode, struct writeback_control *wbc)
>  {
> +       struct fuse_conn *fc = get_fuse_conn(inode);
>         struct fuse_inode *fi = get_fuse_inode(inode);
>         struct fuse_file *ff;
>         int err;
>
> +       /*
> +        * Kernel c/mtime should not be updated to the server in the
> +        * writeback_cache_v2 mode as server c/mtime are official.
> +        */
> +       if (fc->writeback_cache_v2)
> +               return 0;
> +
>         /*
>          * Inode is always written before the last reference is dropped and
>          * hence this should not be reached from reclaim.
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 488b460e046f..47de36146fb8 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -654,6 +654,9 @@ struct fuse_conn {
>         /* show legacy mount options */
>         unsigned int legacy_opts_show:1;
>
> +       /* Improved writeback cache policy */
> +       unsigned writeback_cache_v2:1;
> +

Seeing that writeback_cache_v2 depends on writeback_cache
I wonder whether that will not be better represented as:

        /** write-back cache policy (default is write-through) */
-       unsigned writeback_cache:1;
+      unsigned writeback_cache:2;


Looking at the recently added handle_killpriv_v2, I also wonder
if that would not have been better.

Vivek,
is handle_killpriv_v2 really independent of handle_killpriv?
Seeing test like these worry me as they are inviting bugs:

                if (!fc->handle_killpriv && !fc->handle_killpriv_v2) {

Thanks,
Amir.

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