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[v2] ceph: fix blindly expanding the readahead windows

Message ID 20230504052306.405208-1-xiubli@redhat.com
State New
Headers show
Series [v2] ceph: fix blindly expanding the readahead windows | expand

Commit Message

Xiubo Li May 4, 2023, 5:23 a.m. UTC 
From: Xiubo Li <xiubli@redhat.com>

Blindly expanding the readahead windows will cause unneccessary
pagecache thrashing and also will introdue the network workload.
We should disable expanding the windows if the readahead is disabled
and also shouldn't expand the windows too much.

Expanding forward firstly instead of expanding backward for possible
sequential reads.

URL: https://www.spinics.net/lists/ceph-users/msg76183.html
Signed-off-by: Xiubo Li <xiubli@redhat.com>
---

V2:
- fix possible cross-block issue pointed out by Ilya.


 fs/ceph/addr.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)
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Patch

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index ca4dc6450887..03a326da8fd8 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -188,16 +188,28 @@  static void ceph_netfs_expand_readahead(struct netfs_io_request *rreq)
 	struct inode *inode = rreq->inode;
 	struct ceph_inode_info *ci = ceph_inode(inode);
 	struct ceph_file_layout *lo = &ci->i_layout;
+	unsigned long max_pages = inode->i_sb->s_bdi->ra_pages;
+	unsigned long max_len = max_pages << PAGE_SHIFT;
+	unsigned long len;
 	u32 blockoff;
 	u64 blockno;
 
-	/* Expand the start downward */
-	blockno = div_u64_rem(rreq->start, lo->stripe_unit, &blockoff);
-	rreq->start = blockno * lo->stripe_unit;
-	rreq->len += blockoff;
+	/* Readahead is disabled */
+	if (!max_pages)
+		return;
+
+	/* Try to expand the length forward by rounding up it to the next block */
+	div_u64_rem(rreq->start + rreq->len, lo->stripe_unit, &blockoff);
+	len = lo->stripe_unit - blockoff;
+	if (rreq->len + len <= max_len)
+		rreq->len += len;
 
-	/* Now, round up the length to the next block */
-	rreq->len = roundup(rreq->len, lo->stripe_unit);
+	/* Try to expand the start downward */
+	blockno = div_u64_rem(rreq->start, lo->stripe_unit, &blockoff);
+	if (rreq->len + blockoff <= max_len) {
+		rreq->start = blockno * lo->stripe_unit;
+		rreq->len += blockoff;
+	}
 }
 
 static bool ceph_netfs_clamp_length(struct netfs_io_subrequest *subreq)