[PATCH v11 02/22] cachefiles: notify the user daemon when looking up cookie

[David Howells]

Jeffle Xu <jefflexu at linux.alibaba.com> wrote:

> Fscache/CacheFiles used to serve as a local cache for a remote
> networking fs. A new on-demand read mode will be introduced for
> CacheFiles, which can boost the scenario where on-demand read semantics
> are needed, e.g. container image distribution.
> 
> The essential difference between these two modes is seen when a cache
> miss occurs: In the original mode, the netfs will fetch the data from
> the remote server and then write it to the cache file; in on-demand
> read mode, fetching the data and writing it into the cache is delegated
> to a user daemon.
> 
> As the first step, notify the user daemon when looking up cookie. In
> this case, an anonymous fd is sent to the user daemon, through which the
> user daemon can write the fetched data to the cache file. Since the user
> daemon may move the anonymous fd around, e.g. through dup(), an object
> ID uniquely identifying the cache file is also attached.
> 
> Also add one advisory flag (FSCACHE_ADV_WANT_CACHE_SIZE) suggesting that
> the cache file size shall be retrieved at runtime. This helps the
> scenario where one cache file contains multiple netfs files, e.g. for
> the purpose of deduplication. In this case, netfs itself has no idea the
> size of the cache file, whilst the user daemon should give the hint on
> it.
> 
> Signed-off-by: Jeffle Xu <jefflexu at linux.alibaba.com>

Acked-by: David Howells <dhowells at redhat.com>