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, 10 insertions, 6 deletions

diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index bf5c48066fac..b4a0baa46dcc 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -806,12 +806,16 @@ cache in your filesystem.  The following members are defined:
 	object.  The pages are consecutive in the page cache and are
 	locked.  The implementation should decrement the page refcount
 	after starting I/O on each page.  Usually the page will be
-	unlocked by the I/O completion handler.  If the filesystem decides
-	to stop attempting I/O before reaching the end of the readahead
-	window, it can simply return.  The caller will decrement the page
-	refcount and unlock the remaining pages for you.  Set PageUptodate
-	if the I/O completes successfully.  Setting PageError on any page
-	will be ignored; simply unlock the page if an I/O error occurs.
+	unlocked by the I/O completion handler.  The set of pages are
+	divided into some sync pages followed by some async pages,
+	rac->ra->async_size gives the number of async pages.  The
+	filesystem should attempt to read all sync pages but may decide
+	to stop once it reaches the async pages.  If it does decide to
+	stop attempting I/O, it can simply return.  The caller will
+	remove the remaining pages from the address space, unlock them
+	and decrement the page refcount.  Set PageUptodate if the I/O
+	completes successfully.  Setting PageError on any page will be
+	ignored; simply unlock the page if an I/O error occurs.
 
 ``readpages``
 	called by the VM to read pages associated with the address_space