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Date:   Wed, 29 Jun 2022 13:57:42 -0400
From:   Vivek Goyal <vgoyal@redhat.com>
To:     Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Cc:     miklos@szeredi.hu, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, xieyongji@bytedance.com,
        fam.zheng@bytedance.com, Miklos Szeredi <mszeredi@redhat.com>
Subject: Re: [PATCH] fuse: writeback_cache consistency enhancement
 (writeback_cache_v2)
Message-ID: <YrySlhapXqHX4/1P@redhat.com>
References: <20220624055825.29183-1-zhangjiachen.jaycee@bytedance.com>
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In-Reply-To: <20220624055825.29183-1-zhangjiachen.jaycee@bytedance.com>
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On Fri, Jun 24, 2022 at 01:58:25PM +0800, Jiachen Zhang 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.

We have a init flag to negotiate FUSE_AUTO_INVAL_DATA to enable/disable
fc->auto_inval_data. So should it be filesystem's (user space) responsibility
to not use FUSE_AUTO_INVAL_DATA when using FUSE_WRITEBACK_CACHE. I mean,
otherwise it is confusing. On one hand filesystem is using FUSE_AUTO_INVAL_DATA
but it silently is ingored by kernel because FUSE_WRITEBACK_CACHE is
being used as well.

will it make more sense to just document it and let filesystem authors
enable options carefully.

BTW, this probably should be a separate patch in the patch series anyway
with proper explanation.

Thanks
Vivek

> 
>     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);
> +		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
>