cifs: add cifs_async_readv
...which will allow cifs to do an asynchronous read call to the server. The caller will allocate and set up cifs_readdata for each READ_AND_X call that should be issued on the wire. The pages passed in are added to the pagecache, but not placed on the LRU list yet (as we need the page->lru to keep the pages on the list in the readdata). When cifsd identifies the mid, it will see that there is a special receive handler for the call, and use that to receive the rest of the frame. cifs_readv_receive will then marshal up a kvec array with kmapped pages from the pagecache, which eliminates one copy of the data. Once the data is received, the pages are added to the LRU list, set uptodate, and unlocked. Reviewed-and-Tested-by: NPavel Shilovsky <piastry@etersoft.ru> Signed-off-by: NJeff Layton <jlayton@redhat.com>
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