[RFC PATCH] mm : using bdi->ra_pages instead of ra->ra_pages within readahead

[Zhaoyang Huang]

file->f_ra->ra_pages will remain the initialized value since it opend, which may
be NOT equal to bdi->ra_pages as the latter one is updated somehow(etc,
echo xxx > /sys/block/dm/queue/read_ahead_kb).So having readahead use
bdi->ra_pages.

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
---
 mm/filemap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index d78f577..259dcfd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2499,8 +2499,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	 */
 	fpin = maybe_unlock_mmap_for_io(vmf, fpin);
 	ra->start = max_t(long, 0, offset - ra->ra_pages / 2);
-	ra->size = ra->ra_pages;
-	ra->async_size = ra->ra_pages / 4;
+	ra->size = inode_to_bdi(mapping->host)->ra_pages;
+	ra->async_size = ra->size / 4;
 	ra_submit(ra, mapping, file);
 	return fpin;
 }
-- 
1.9.1