mm: share PG_readahead and PG_reclaim (fe3cba17) · Commits · 戴 / test

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>

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);

		/*

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