[RFC PATCH V4] mm readahead: Fix readahead fail for no local memory and limit readahead pages

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

We limit the number of readahead pages to 4k.

max_sane_readahead returns zero on the cpu having no local memory
node. Fix that by returning a sanitized number of pages viz.,
minimum of (requested pages, 4k, number of local free pages)

Result:
fadvise experiment with FADV_WILLNEED on a x240 machine with 1GB testfile
32GB* 4G RAM  numa machine ( 12 iterations) yielded

kernel       Avg        Stddev
base         7.264      0.56%
patched      7.285      1.14%

Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
---
 mm/readahead.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

V4:  incorporated 16MB limit suggested by Linus for readahead and
fixed transitioning to large readahead anomaly pointed by Andrew Morton with
Honza's suggestion.

Test results shows no significant overhead with the current changes.

(Do I have to break patches into two??)

Suggestions/Comments please let me know.

diff --git a/mm/readahead.c b/mm/readahead.c
index 7cdbb44..2f561a0 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -237,14 +237,30 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 	return ret;
 }
 
+#define MAX_REMOTE_READAHEAD   4096UL
 /*
  * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
  * sensible upper limit.
  */
 unsigned long max_sane_readahead(unsigned long nr)
 {
-	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
-		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
+	unsigned long local_free_page;
+	unsigned long sane_nr;
+	int nid;
+
+	nid = numa_node_id();
+	sane_nr = min(nr, MAX_REMOTE_READAHEAD);
+
+	local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
+			  + node_page_state(nid, NR_FREE_PAGES);
+
+	/*
+	 * Readahead onto remote memory is better than no readahead when local
+	 * numa node does not have memory. We sanitize readahead size depending
+	 * on free memory in the local node but limiting to 4k pages.
+	 */
+	return node_present_pages(nid) ?
+				min(sane_nr, local_free_page / 2) : sane_nr;
 }
 
 /*
-- 
1.7.11.7

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