mm: share PG_readahead and PG_reclaim

Share the same page flag bit for PG_readahead and PG_reclaim.

One is used only on file reads, another is only for emergency writes.  One
is used mostly for fresh/young pages, another is for old pages.

Combinations of possible interactions are:

a) clear PG_reclaim => implicit clear of PG_readahead
	it will delay an asynchronous readahead into a synchronous one
	it actually does _good_ for readahead:
		the pages will be reclaimed soon, it's readahead thrashing!
		in this case, synchronous readahead makes more sense.

b) clear PG_readahead => implicit clear of PG_reclaim
	one(and only one) page will not be reclaimed in time
	it can be avoided by checking PageWriteback(page) in readahead first

c) set PG_reclaim => implicit set of PG_readahead
	will confuse readahead and make it restart the size rampup process
	it's a trivial problem, and can mostly be avoided by checking
	PageWriteback(page) first in readahead

d) set PG_readahead => implicit set of PG_reclaim
	PG_readahead will never be set on already cached pages.
	PG_reclaim will always be cleared on dirtying a page.
	so not a problem.

In summary,
	a)   we get better behavior
	b,d) possible interactions can be avoided
	c)   racy condition exists that might affect readahead, but the chance
	     is _really_ low, and the hurt on readahead is trivial.

Compound pages also use PG_reclaim, but for now they do not interact with
reclaim/readahead code.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 709d92fd2877..a454176c3e30 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -83,7 +83,6 @@
 #define PG_private              11      /* If pagecache, has fs-private data */
 
 #define PG_writeback            12      /* Page is under writeback */
-#define PG_readahead            13      /* Reminder to do async read-ahead */
 #define PG_compound             14      /* Part of a compound page */
 #define PG_swapcache            15      /* Swap page: swp_entry_t in private */
 
@@ -91,6 +90,9 @@
 #define PG_reclaim              17      /* To be reclaimed asap */
 #define PG_buddy                19      /* Page is free, on buddy lists */
 
+/* PG_readahead is only used for file reads; PG_reclaim is only for writes */
+#define PG_readahead            PG_reclaim /* Reminder to do async read-ahead */
+
 /* PG_owner_priv_1 users should have descriptive aliases */
 #define PG_checked              PG_owner_priv_1 /* Used by some filesystems */
 #define PG_pinned               PG_owner_priv_1 /* Xen pinned pagetable */