Introduce a readahead flags field and embed the existing mmap_miss in it
(to save space).
It will be possible to lose the flags in race conditions, however the
impact should be limited.
CC: Nick Piggin <npiggin@xxxxxxx>
CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
CC: Steven Whitehouse <swhiteho@xxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
include/linux/fs.h | 30 +++++++++++++++++++++++++++++-
mm/filemap.c | 7 ++-----
2 files changed, 31 insertions(+), 6 deletions(-)
--- linux.orig/include/linux/fs.h 2009-12-30 11:07:04.000000000 +0800
+++ linux/include/linux/fs.h 2009-12-30 12:59:28.000000000 +0800
@@ -889,10 +889,38 @@ struct file_ra_state {
there are only # of pages ahead */
unsigned int ra_pages; /* Maximum readahead window */
- unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
+ unsigned int flags;
loff_t prev_pos; /* Cache last read() position */
};
+/* low 16 bits: cache miss stat for mmap accesses */
+#define RA_FLAG_MMAP_MISS 0x0000ffff
+
+/*
+ * Don't do flags++ directly to avoid possible overflow:
+ * the ra fields can be accessed concurrently in a racy way.
+ */
+static inline unsigned int ra_mmap_miss_inc(struct file_ra_state *ra)
+{
+ unsigned int miss = ra->flags & RA_FLAG_MMAP_MISS;
+
+ if (miss < RA_FLAG_MMAP_MISS) {
+ miss++;
+ ra->flags = miss | (ra->flags &~ RA_FLAG_MMAP_MISS);
+ }
+ return miss;
+}
+
+static inline void ra_mmap_miss_dec(struct file_ra_state *ra)
+{
+ unsigned int miss = ra->flags & RA_FLAG_MMAP_MISS;
+
+ if (miss) {
+ miss--;
+ ra->flags = miss | (ra->flags &~ RA_FLAG_MMAP_MISS);
+ }
+}
+
/*
* Check if @index falls in the readahead windows.
*/
--- linux.orig/mm/filemap.c 2009-12-30 11:07:04.000000000 +0800
+++ linux/mm/filemap.c 2009-12-30 11:07:22.000000000 +0800
@@ -1418,14 +1418,12 @@ static void do_sync_mmap_readahead(struc
return;
}
- if (ra->mmap_miss < INT_MAX)
- ra->mmap_miss++;
/*
* Do we miss much more than hit in this file? If so,
* stop bothering with read-ahead. It will only hurt.
*/
- if (ra->mmap_miss > MMAP_LOTSAMISS)
+ if (ra_mmap_miss_inc(ra) > MMAP_LOTSAMISS)
return;
/*
@@ -1455,8 +1453,7 @@ static void do_async_mmap_readahead(stru
/* If we don't want any read-ahead, don't bother */
if (VM_RandomReadHint(vma))
return;
- if (ra->mmap_miss > 0)
- ra->mmap_miss--;
+ ra_mmap_miss_dec(ra);
if (PageReadahead(page))
page_cache_async_readahead(mapping, ra, file,
page, offset, ra->ra_pages);
--
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