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| author | Fengguang Wu <wfg@mail.ustc.edu.cn> | 2007-07-19 12:48:07 +0400 |
|---|---|---|
| committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-07-19 21:04:44 +0400 |
| commit | fe3cba17c49471e99d3421e675fc8b3deaaf0b70 (patch) | |
| tree | df696c4584c6db2e439f068d2474fcb946ca587d /include/linux/page-flags.h | |
| parent | d8983910a4045fa21022cfccf76ed13eb40fd7f5 (diff) | |
| download | linux-fe3cba17c49471e99d3421e675fc8b3deaaf0b70.tar.xz | |
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>
Diffstat (limited to 'include/linux/page-flags.h')
| -rw-r--r-- | include/linux/page-flags.h | 4 |
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 */ |