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Subject[PATCH] properly account readahead file major faults
Hi,

The fault accounting of filemap_dopage() is currently unable to account
for readahead pages as major faults.

Which means that getrusage's major fault reporting is pretty useless.

Fix that by using the PageReferenced and PageActive bits: allows
differentiation between newly accessed pages and reaccessed pages.

Follows the output of "/usr/bin/time -v" of an app which reads
10MB through a mapping on cold pagecache.

BEFORE PATCH:
Size: 10MB
Loops: 10
opening ./scan-testfile
reading scan-testfile, length = 10MB
Command being timed: "./scan-shared 10 10"
<snip>
Major (requiring I/O) page faults: 225
Minor (reclaiming a frame) page faults: 2437

AFTER PATCH:
Size: 10MB
Loops: 10
opening ./scan-testfile
reading scan-testfile, length = 10MB
Command being timed: "./scan-shared 10 10"
<>snip>
Major (requiring I/O) page faults: 2562
Minor (reclaiming a frame) page faults: 101



Signed-off-by: Marcelo Tosatti <marcelo.tosatti@cyclades.com>

diff --git a/mm/filemap.c b/mm/filemap.c
index 5d6e4c2..8655443 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1237,14 +1237,6 @@ retry_find:
if (ra->mmap_miss > ra->mmap_hit + MMAP_LOTSAMISS)
goto no_cached_page;

- /*
- * To keep the pgmajfault counter straight, we need to
- * check did_readaround, as this is an inner loop.
- */
- if (!did_readaround) {
- majmin = VM_FAULT_MAJOR;
- inc_page_state(pgmajfault);
- }
did_readaround = 1;
ra_pages = max_sane_readahead(file->f_ra.ra_pages);
if (ra_pages) {
@@ -1273,6 +1265,15 @@ success:
/*
* Found the page and have a reference on it.
*/
+ if (PageActive(page) && !PageReferenced(page)) {
+ /* only account active pages as major faults, since inactive
+ * pages might have their referenced bit cleaned by
+ * memory scanning.
+ */
+ majmin = VM_FAULT_MAJOR;
+ inc_page_state(pgmajfault);
+ }
+
mark_page_accessed(page);
if (type)
*type = majmin;
@@ -1312,10 +1313,6 @@ no_cached_page:
return NULL;

page_not_uptodate:
- if (!did_readaround) {
- majmin = VM_FAULT_MAJOR;
- inc_page_state(pgmajfault);
- }
lock_page(page);

/* Did it get unhashed while we waited for it? */
-
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