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Commit fe3cba17 authored by Fengguang Wu's avatar Fengguang Wu Committed by Linus Torvalds
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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: default avatarFengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d8983910

include/linux/page-flags.h

+3 −1
Original line number Diff line number Diff line
@@ -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 */

mm/page-writeback.c

+1 −0
Original line number Diff line number Diff line
@@ -920,6 +920,7 @@ int clear_page_dirty_for_io(struct page *page) 


	BUG_ON(!PageLocked(page));



	ClearPageReclaim(page);

	if (mapping && mapping_cap_account_dirty(mapping)) {

		/*

		 * Yes, Virginia, this is indeed insane.

mm/page_alloc.c

+0 −7
Original line number Diff line number Diff line
@@ -453,12 +453,6 @@ static inline int free_pages_check(struct page *page) 
			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 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags) 
			1 << PG_locked	|

			1 << PG_active	|

			1 << PG_dirty	|

			1 << PG_reclaim	|

			1 << PG_slab    |

			1 << PG_swapcache |

			1 << PG_writeback |

mm/readahead.c

+6 −0
Original line number Diff line number Diff line
@@ -448,6 +448,12 @@ page_cache_readahead_ondemand(struct address_space *mapping, 
		return 0;



	if (page) {

		/*

		 * It can be PG_reclaim.

		 */

		if (PageWriteback(page))

			return 0;



		ClearPageReadahead(page);



		/*