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>
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 709d92f..a454176 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 */
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e624827..51b3eb6 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -920,6 +920,7 @@
BUG_ON(!PageLocked(page));
+ ClearPageReclaim(page);
if (mapping && mapping_cap_account_dirty(mapping)) {
/*
* Yes, Virginia, this is indeed insane.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2165be9..43cb3b3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -453,12 +453,6 @@
1 << PG_reserved |
1 << PG_buddy ))))
bad_page(page);
- /*
- * PageReclaim == PageTail. It is only an error
- * for PageReclaim to be set if PageCompound is clear.
- */
- if (unlikely(!PageCompound(page) && PageReclaim(page)))
- bad_page(page);
if (PageDirty(page))
__ClearPageDirty(page);
/*
@@ -602,7 +596,6 @@
1 << PG_locked |
1 << PG_active |
1 << PG_dirty |
- 1 << PG_reclaim |
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback |
diff --git a/mm/readahead.c b/mm/readahead.c
index 5b3c9b7..205a4a4 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -448,6 +448,12 @@
return 0;
if (page) {
+ /*
+ * It can be PG_reclaim.
+ */
+ if (PageWriteback(page))
+ return 0;
+
ClearPageReadahead(page);
/*