Re: [PATCH 1/2] fscache,cachefiles: add prepare_ondemand_read() callback

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2022-11-04 at 15:26 +0800, Jingbo Xu wrote:
> Add prepare_ondemand_read() callback dedicated for the on-demand read
> scenario, so that callers from this scenario can be decoupled from
> netfs_io_subrequest.
> 
> To reuse the hole detecting logic as mush as possible, both the
> implementation of prepare_read() and prepare_ondemand_read() inside
> Cachefiles call a common routine.
> 
> In the near future, prepare_read() will get enhanced and more
> information will be needed and then returned to callers. Thus
> netfs_io_subrequest is a reasonable candidate for holding places for all
> these information needed in the internal implementation.
> 
> Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
> ---
>  fs/cachefiles/io.c                | 42 +++++++++++++++++++++++++------
>  include/linux/netfs.h             |  7 ++++++
>  include/trace/events/cachefiles.h |  4 +--
>  3 files changed, 43 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/cachefiles/io.c b/fs/cachefiles/io.c
> index 000a28f46e59..6427259fcba9 100644
> --- a/fs/cachefiles/io.c
> +++ b/fs/cachefiles/io.c
> @@ -385,16 +385,11 @@ static int cachefiles_write(struct netfs_cache_resources *cres,
>  				  term_func, term_func_priv);
>  }
>  
> -/*
> - * Prepare a read operation, shortening it to a cached/uncached
> - * boundary as appropriate.
> - */
> -static enum netfs_io_source cachefiles_prepare_read(struct netfs_io_subrequest *subreq,
> -						      loff_t i_size)
> +static enum netfs_io_source cachefiles_do_prepare_read(struct netfs_io_subrequest *subreq,
> +						       struct netfs_cache_resources *cres,
> +						       loff_t i_size)
>  {
>  	enum cachefiles_prepare_read_trace why;
> -	struct netfs_io_request *rreq = subreq->rreq;
> -	struct netfs_cache_resources *cres = &rreq->cache_resources;
>  	struct cachefiles_object *object;
>  	struct cachefiles_cache *cache;
>  	struct fscache_cookie *cookie = fscache_cres_cookie(cres);
> @@ -501,6 +496,36 @@ static enum netfs_io_source cachefiles_prepare_read(struct netfs_io_subrequest *
>  	return ret;
>  }
>  
> +/*
> + * Prepare a read operation, shortening it to a cached/uncached
> + * boundary as appropriate.
> + */
> +static enum netfs_io_source cachefiles_prepare_read(struct netfs_io_subrequest *subreq,
> +						      loff_t i_size)
> +{
> +	return cachefiles_do_prepare_read(subreq,
> +			&subreq->rreq->cache_resources, i_size);
> +}
> +
> +/*
> + * Prepare an on-demand read operation, shortening it to a cached/uncached
> + * boundary as appropriate.
> + */
> +static enum netfs_io_source cachefiles_prepare_ondemand_read(struct netfs_cache_resources *cres,
> +		loff_t start, size_t *_len, loff_t i_size)
> +{
> +	enum netfs_io_source source;
> +	struct netfs_io_subrequest subreq = {
> +		.start	= start,
> +		.len	= *_len,
> +		.flags	= 1 << NETFS_SREQ_ONDEMAND,
> +	};
> +

Faking up a struct like this is sort of fragile. What if we change
cachefiles_do_prepare_read to consult other fields in this structure
later?

It might be best to have cachefiles_do_prepare_read take individual
start, len, and flags values instead of doing this.


> +	source = cachefiles_do_prepare_read(&subreq, cres, i_size);
> +	*_len = subreq.len;
> +	return source;
> +}
> +
>  /*
>   * Prepare for a write to occur.
>   */
> @@ -621,6 +646,7 @@ static const struct netfs_cache_ops cachefiles_netfs_cache_ops = {
>  	.write			= cachefiles_write,
>  	.prepare_read		= cachefiles_prepare_read,
>  	.prepare_write		= cachefiles_prepare_write,
> +	.prepare_ondemand_read	= cachefiles_prepare_ondemand_read,
>  	.query_occupancy	= cachefiles_query_occupancy,
>  };
>  
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index f2402ddeafbf..d82071c37133 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -267,6 +267,13 @@ struct netfs_cache_ops {
>  			     loff_t *_start, size_t *_len, loff_t i_size,
>  			     bool no_space_allocated_yet);
>  
> +	/* Prepare an on-demand read operation, shortening it to a cached/uncached
> +	 * boundary as appropriate.
> +	 */
> +	enum netfs_io_source (*prepare_ondemand_read)(struct netfs_cache_resources *cres,
> +						      loff_t start, size_t *_len,
> +						      loff_t i_size);
> +
>  	/* Query the occupancy of the cache in a region, returning where the
>  	 * next chunk of data starts and how long it is.
>  	 */
> diff --git a/include/trace/events/cachefiles.h b/include/trace/events/cachefiles.h
> index d8d4d73fe7b6..655d5900b8ef 100644
> --- a/include/trace/events/cachefiles.h
> +++ b/include/trace/events/cachefiles.h
> @@ -448,14 +448,14 @@ TRACE_EVENT(cachefiles_prep_read,
>  			     ),
>  
>  	    TP_fast_assign(
> -		    __entry->rreq	= sreq->rreq->debug_id;
> +		    __entry->rreq	= sreq->rreq ? sreq->rreq->debug_id : 0;
>  		    __entry->index	= sreq->debug_index;
>  		    __entry->flags	= sreq->flags;
>  		    __entry->source	= source;
>  		    __entry->why	= why;
>  		    __entry->len	= sreq->len;
>  		    __entry->start	= sreq->start;
> -		    __entry->netfs_inode = sreq->rreq->inode->i_ino;
> +		    __entry->netfs_inode = sreq->rreq ? sreq->rreq->inode->i_ino : 0;
>  		    __entry->cache_inode = cache_inode;
>  			   ),
>  

-- 
Jeff Layton <jlayton@kernel.org>