[PATCH 2/3] readahead: fadvise drop behind controls

[Peter Zijlstra]

currently drop behind is controlled by a heuristic:

  ra->size == ra->ra_pages

provide some fadvise() control:

POSIX_FADV_NORMAL     - heuristic
POSIX_FADV_RANDOM     - always disable drop behind
POSIX_FADV_SEQUENTIAL - always enable drop behind

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 include/linux/fs.h |    7 +++++++
 mm/fadvise.c       |    5 +++++
 mm/readahead.c     |    4 +++-
 3 files changed, 15 insertions(+), 1 deletion(-)

Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -705,8 +705,15 @@ struct file_ra_state {
 	unsigned int ra_pages;		/* Maximum readahead window */
 	int mmap_miss;			/* Cache miss stat for mmap accesses */
 	loff_t prev_pos;		/* Cache last read() position */
+	unsigned int flags;		/* readahead flags */
 };
 
+#define F_RA_DROP_BEHIND_NEVER		1
+#define F_RA_DROP_BEHIND_ALWAYS		2
+
+#define F_RA_DROP_BEHIND_MASK \
+	(F_RA_DROP_BEHIND_NEVER|F_RA_DROP_BEHIND_ALWAYS)
+
 /*
  * Check if @index falls in the readahead windows.
  */
Index: linux-2.6/mm/fadvise.c
===================================================================
--- linux-2.6.orig/mm/fadvise.c
+++ linux-2.6/mm/fadvise.c
@@ -65,12 +65,17 @@ asmlinkage long sys_fadvise64_64(int fd,
 	switch (advice) {
 	case POSIX_FADV_NORMAL:
 		file->f_ra.ra_pages = bdi->ra_pages;
+		file->f_ra.flags &= ~F_RA_DROP_BEHIND_MASK;
 		break;
 	case POSIX_FADV_RANDOM:
 		file->f_ra.ra_pages = 0;
+		file->f_ra.flags &= ~F_RA_DROP_BEHIND_MASK;
+		file->f_ra.flags |= F_RA_DROP_BEHIND_NEVER;
 		break;
 	case POSIX_FADV_SEQUENTIAL:
 		file->f_ra.ra_pages = bdi->ra_pages * 2;
+		file->f_ra.flags &= ~F_RA_DROP_BEHIND_MASK;
+		file->f_ra.flags |= F_RA_DROP_BEHIND_ALWAYS;
 		break;
 	case POSIX_FADV_WILLNEED:
 		if (!mapping->a_ops->readpage) {
Index: linux-2.6/mm/readahead.c
===================================================================
--- linux-2.6.orig/mm/readahead.c
+++ linux-2.6/mm/readahead.c
@@ -485,7 +485,9 @@ page_cache_async_readahead(struct addres
 	 * hint that there is sequential IO, which implies that the pages that
 	 * have been used thus far can be reclaimed
 	 */
-	if (ra->size == ra->ra_pages) do {
+	if (!(ra->flags & F_RA_DROP_BEHIND_NEVER) &&
+	    ((ra->flags & F_RA_DROP_BEHIND_ALWAYS) ||
+	     ra->size == ra->ra_pages)) do {
 		nr_pages = find_get_pages(mapping,
 				demote_idx, ARRAY_SIZE(pages), pages);
 

--

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