mm: document and polish read-ahead code

Add some "big-picture" documentation for read-ahead and polish the code
to make it fit this documentation.

The meaning of ->async_size is clarified to match its name.  i.e.  Any
request to ->readahead() has a sync part and an async part.  The caller
will wait for the sync pages to complete, but will not wait for the
async pages.  The first async page is still marked PG_readahead

Note that the current function names page_cache_sync_ra() and
page_cache_async_ra() are misleading.  All ra request are partly sync
and partly async, so either part can be empty.  A page_cache_sync_ra()
request will usually set ->async_size non-zero, implying it is not all
synchronous.

When a non-zero req_count is passed to page_cache_async_ra(), the
implication is that some prefix of the request is synchronous, though
the calculation made there is incorrect - I haven't tried to fix it.

Link: https://lkml.kernel.org/r/164549983734.9187.11586890887006601405.stgit@noble.brown
Signed-off-by: NeilBrown <neilb@suse.de>
Cc: Anna Schumaker <Anna.Schumaker@Netapp.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Paolo Valente <paolo.valente@linaro.org>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e2d892b201b0..8b5c486bd4a2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -930,10 +930,15 @@ struct fown_struct {
  * struct file_ra_state - Track a file's readahead state.
  * @start: Where the most recent readahead started.
  * @size: Number of pages read in the most recent readahead.
- * @async_size: Start next readahead when this many pages are left.
- * @ra_pages: Maximum size of a readahead request.
+ * @async_size: Numer of pages that were/are not needed immediately
+ *      and so were/are genuinely "ahead".  Start next readahead when
+ *      the first of these pages is accessed.
+ * @ra_pages: Maximum size of a readahead request, copied from the bdi.
  * @mmap_miss: How many mmap accesses missed in the page cache.
  * @prev_pos: The last byte in the most recent read request.
+ *
+ * When this structure is passed to ->readahead(), the "most recent"
+ * readahead means the current readahead.
  */
 struct file_ra_state {
 	pgoff_t start;