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

[Miklos Szeredi <miklos@szeredi.hu>]

On Fri, 24 Jun 2022 at 07:58, Jiachen Zhang
<zhangjiachen.jaycee@bytedance.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.

Interesting.  The issue there is that ctime/mtime would change
spontaneously when data is flushed to server.  Maybe that's a lesser
of the evils, I don't know...

>
>     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.

Right, I think auto_inval_data has always been broken with writeback_cache.
>
>     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@redhat.com>
> Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.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;
> +
>         /*
>          * fs kills suid/sgid/cap on write/chown/trunc. suid is killed on
>          * write/trunc only if caller did not have CAP_FSETID.  sgid is killed
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 8c0665c5dff8..2d5fa82b08b6 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -237,14 +237,41 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
>         u32 cache_mask;
>         loff_t oldsize;
>         struct timespec64 old_mtime;
> +       bool try_wb_update = false;
> +
> +       if (fc->writeback_cache_v2 && S_ISREG(inode->i_mode)) {
> +               inode_lock(inode);

I don't think this can work.   fuse_change_attributes() might be
called from within inlode locked context.  E.g.

lookup_slow -> __lookup_slow -> d_revalidate -> fuse_dentry_revalidate
-> fuse_change_attributes

> +               try_wb_update = true;
> +       }
>
>         spin_lock(&fi->lock);
>         /*
>          * In case of writeback_cache enabled, writes update mtime, ctime and
>          * may update i_size.  In these cases trust the cached value in the
>          * inode.
> +        *
> +        * In writeback_cache_v2 mode, if all the following conditions are met,
> +        * then we allow the attributes to be refreshed:
> +        *
> +        * - inode is not in the process of being written (I_SYNC)
> +        * - inode has no dirty pages (I_DIRTY_PAGES)
> +        * - inode data-related attributes are clean (I_DIRTY_DATASYNC)
> +        * - inode does not have any page writeback in progress
> +        *
> +        * Note: checking PAGECACHE_TAG_WRITEBACK is not sufficient in fuse,
> +        * since inode can appear to have no PageWriteback pages, yet still have
> +        * outstanding write request.
>          */
>         cache_mask = fuse_get_cache_mask(inode);
> +       if (try_wb_update && !(inode->i_state & (I_DIRTY_PAGES | I_SYNC |
> +           I_DIRTY_DATASYNC)) && RB_EMPTY_ROOT(&fi->writepages))
> +               cache_mask &= ~(STATX_MTIME | STATX_CTIME | STATX_SIZE);
> +       /*
> +        * Ensure the ordering of cleanness checking and following attr_version
> +        * comparison.
> +        */
> +       smp_mb();
> +
>         if (cache_mask & STATX_SIZE)
>                 attr->size = i_size_read(inode);
>
> @@ -283,7 +310,13 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
>                         truncate_pagecache(inode, attr->size);
>                         if (!fc->explicit_inval_data)
>                                 inval = true;
> -               } else if (fc->auto_inval_data) {
> +               } else if (!fc->writeback_cache_v2 && fc->auto_inval_data) {
> +                       /*
> +                        * When fc->writeback_cache_v2 is set, the old_mtime
> +                        * can be generated by kernel and must not equal to
> +                        * new_mtime generated by server. So skip in such
> +                        * case.
> +                        */
>                         struct timespec64 new_mtime = {
>                                 .tv_sec = attr->mtime,
>                                 .tv_nsec = attr->mtimensec,
> @@ -303,6 +336,9 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
>
>         if (IS_ENABLED(CONFIG_FUSE_DAX))
>                 fuse_dax_dontcache(inode, attr->flags);
> +
> +       if (try_wb_update)
> +               inode_unlock(inode);
>  }
>
>  static void fuse_init_inode(struct inode *inode, struct fuse_attr *attr)
> @@ -1153,6 +1189,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
>                                 fc->async_dio = 1;
>                         if (flags & FUSE_WRITEBACK_CACHE)
>                                 fc->writeback_cache = 1;
> +                       if (flags & FUSE_WRITEBACK_CACHE_V2) {
> +                               fc->writeback_cache = 1;
> +                               fc->writeback_cache_v2 = 1;
> +                       }
>                         if (flags & FUSE_PARALLEL_DIROPS)
>                                 fc->parallel_dirops = 1;
>                         if (flags & FUSE_HANDLE_KILLPRIV)
> @@ -1234,7 +1274,7 @@ void fuse_send_init(struct fuse_mount *fm)
>                 FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
>                 FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
>                 FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
> -               FUSE_SECURITY_CTX;
> +               FUSE_SECURITY_CTX | FUSE_WRITEBACK_CACHE_V2;
>  #ifdef CONFIG_FUSE_DAX
>         if (fm->fc->dax)
>                 flags |= FUSE_MAP_ALIGNMENT;
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index d6ccee961891..b474763bcf59 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -194,6 +194,7 @@
>   *  - add FUSE_SECURITY_CTX init flag
>   *  - add security context to create, mkdir, symlink, and mknod requests
>   *  - add FUSE_HAS_INODE_DAX, FUSE_ATTR_DAX
> + *  - add FUSE_WRITEBACK_CACHE_V2 init flag
>   */
>
>  #ifndef _LINUX_FUSE_H
> @@ -353,6 +354,9 @@ struct fuse_file_lock {
>   * FUSE_SECURITY_CTX:  add security context to create, mkdir, symlink, and
>   *                     mknod
>   * FUSE_HAS_INODE_DAX:  use per inode DAX
> + * FUSE_WRITEBACK_CACHE_V2:
> + *                     allow time/size to be refreshed if no pending write
> + *                     c/mtime not updated from kernel to server
>   */
>  #define FUSE_ASYNC_READ                (1 << 0)
>  #define FUSE_POSIX_LOCKS       (1 << 1)
> @@ -389,6 +393,7 @@ struct fuse_file_lock {
>  /* bits 32..63 get shifted down 32 bits into the flags2 field */
>  #define FUSE_SECURITY_CTX      (1ULL << 32)
>  #define FUSE_HAS_INODE_DAX     (1ULL << 33)
> +#define FUSE_WRITEBACK_CACHE_V2        (1ULL << 34)
>
>  /**
>   * CUSE INIT request/reply flags
> --
> 2.20.1
>