Re: [PATCH v2 18/19] netfs: Keep track of the actual remote file size

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-03-08 at 23:29 +0000, David Howells wrote:
> Provide a place in which to keep track of the actual remote file size in
> the netfs context.  This is needed because inode->i_size will be updated as
> we buffer writes in the pagecache, but the server file size won't get
> updated until we flush them back.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-cachefs@redhat.com
> 
> Link: https://lore.kernel.org/r/164623013727.3564931.17659955636985232717.stgit@warthog.procyon.org.uk/ # v1
> ---
> 
>  include/linux/netfs.h |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index 8458b30172a5..c7bf1eaf51d5 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -126,6 +126,7 @@ struct netfs_i_context {
>  #if IS_ENABLED(CONFIG_FSCACHE)
>  	struct fscache_cookie	*cache;
>  #endif
> +	loff_t			remote_i_size;	/* Size of the remote file */
>  };
>  
>  /*
> @@ -324,6 +325,21 @@ static inline void netfs_i_context_init(struct inode *inode,
>  
>  	memset(ctx, 0, sizeof(*ctx));
>  	ctx->ops = ops;
> +	ctx->remote_i_size = i_size_read(inode);
> +}
> +
> +/**
> + * netfs_resize_file - Note that a file got resized
> + * @inode: The inode being resized
> + * @new_i_size: The new file size
> + *
> + * Inform the netfs lib that a file got resized so that it can adjust its state.
> + */
> +static inline void netfs_resize_file(struct inode *inode, loff_t new_i_size)
> +{
> +	struct netfs_i_context *ctx = netfs_i_context(inode);
> +
> +	ctx->remote_i_size = new_i_size;
>  }
>  
>  /**
> 
> 

This seems like something useful, but I wonder if it'll need some sort
of serialization vs. concurrent updates. Can we assume that netfs itself
will never call netfs_resize_file?

Ceph already has some pretty complicated size tracking, since it can get
async notifications of size changes from the MDS. We'll have to consider
how to integrate this with what it does. Probably this will replace one
(or more?) of its fields.

We may need to offer up some guidance wrt locking.

Reviewed-by: Jeff Layton <jlayton@kernel.org>