[PATCH] readahead: Update the file_ra_state.ra_pages with each readahead operation

[Youling Tang <youling.tang@outlook.com>]

From: Youling Tang <tangyouling@kylinos.cn>

Changing the read_ahead_kb value midway through a sequential read of a
large file found that the ra->ra_pages value remained unchanged (new
ra_pages can only be detected the next time the file is opened). Because
file_ra_state_init() is only called once in do_dentry_open() in most
cases.

In ondemand_readahead(), update bdi->ra_pages to ra->ra_pages to ensure
that the maximum pages that can be allocated by the readahead algorithm
are the same as (read_ahead_kb * 1024) / PAGE_SIZE after read_ahead_kb
is modified.

Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
---
 mm/readahead.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index e815c114de21..3dbabf819187 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -554,12 +554,14 @@ static void ondemand_readahead(struct readahead_control *ractl,
 {
 	struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
 	struct file_ra_state *ra = ractl->ra;
-	unsigned long max_pages = ra->ra_pages;
+	unsigned long max_pages;
 	unsigned long add_pages;
 	pgoff_t index = readahead_index(ractl);
 	pgoff_t expected, prev_index;
 	unsigned int order = folio ? folio_order(folio) : 0;
 
+	max_pages = ra->ra_pages = bdi->ra_pages;
+
 	/*
 	 * If the request exceeds the readahead window, allow the read to
 	 * be up to the optimal hardware IO size
-- 
2.25.1