Re: Re: Re: [RFC PATCH] fuse: support cache revalidation in writeback_cache mode

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Apr 26, 2022 at 03:09:05PM +0200, Miklos Szeredi wrote:
> On Mon, Apr 25, 2022 at 09:52:44PM +0800, Jiachen Zhang wrote:
> 
> > Some users may want both the high performance of writeback 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.
> 
> Okay.
> 
> Here's a preliminary patch that you could try.
> 
> Thanks,
> Miklos
> 
> ---
>  fs/fuse/dir.c             |   35 ++++++++++++++++++++++-------------
>  fs/fuse/file.c            |   17 +++++++++++++++--
>  fs/fuse/fuse_i.h          |   14 +++++++++++++-
>  fs/fuse/inode.c           |   32 +++++++++++++++++++++++++++-----
>  include/uapi/linux/fuse.h |    5 +++++
>  5 files changed, 82 insertions(+), 21 deletions(-)
> 
> --- 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
> + * 			- time/size not cached for falocate/copy_file_range
>   */
>  #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
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -222,19 +222,37 @@ void fuse_change_attributes_common(struc
>  u32 fuse_get_cache_mask(struct inode *inode)
>  {
>  	struct fuse_conn *fc = get_fuse_conn(inode);
> +	struct fuse_inode *fi = get_fuse_inode(inode);
>  
>  	if (!fc->writeback_cache || !S_ISREG(inode->i_mode))
>  		return 0;
>  
> +	/*
> +	 * In writeback_cache_v2 mode if all the following conditions are met,
> +	 * then allow the attributes to be refreshed:
> +	 *
> +	 * - inode is not dirty (I_DIRTY_INODE)
> +	 * - inode is not in the process of being written (I_SYNC)
> +	 * - inode has no dirty pages (I_DIRTY_PAGES)
> +	 * - 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.
> +	 */

Hi,

I started following this thread just now after Jiachen pointed me to
previous conversations. Without going into too much details.

Based on above description, so we will update mtime/ctime/i_size only
if inode does not have dirty pages or nothing is in progress. So that
means sometime we will update it and other times we will ignore it. 

Do I understand it correctly. I am wondering how that is useful to
applications. 

I thought that other remote filesystems might have leasing for this so
that one client can acquire the lease and cache changes and when lease
is broken, this client pushes out all the changes and other client gets
the lease.

Given we don't have any lease mechanism, we probably need to define the
semantics more clearly and we should probably document it as well.

Thanks
Vivek