Re: [PATCH] fuse: writeback_cache consistency enhancement (writeback_cache_v2)

From: Amir Goldstein
Date: Tue Jun 28 2022 - 06:10:13 EST


On Fri, Jun 24, 2022 at 9:03 AM Jiachen Zhang
<zhangjiachen.jaycee@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> [2] https://lore.kernel.org/linux-fsdevel/20220608104202.19461-1-zhangjiachen.jaycee@xxxxxxxxxxxxx/
>
> Suggested-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx>
> ---
> 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.