afs: Fix TTL on VL server and address lists

Currently the TTL on VL server and address lists isn't set in all
circumstances and may be set to poor choices in others, since the TTL is
derived from the SRV/AFSDB DNS record if and when available.

Fix the TTL by limiting the range to a minimum and maximum from the current
time.  At some point these can be made into sysctl knobs.  Further, use the
TTL we obtained from the upcall to set the expiry on negative results too;
in future a mechanism can be added to force reloading of such data.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 22 insertions, 4 deletions

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index 963b6fa51fdf..cf445dbd5f2e 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -20,6 +20,8 @@
 #include "internal.h"
 
 static unsigned __read_mostly afs_cell_gc_delay = 10;
+static unsigned __read_mostly afs_cell_min_ttl = 10 * 60;
+static unsigned __read_mostly afs_cell_max_ttl = 24 * 60 * 60;
 
 static void afs_manage_cell(struct work_struct *);
 
@@ -171,6 +173,8 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 
 		rcu_assign_pointer(cell->vl_servers, vllist);
 		cell->dns_expiry = TIME64_MAX;
+	} else {
+		cell->dns_expiry = ktime_get_real_seconds();
 	}
 
 	_leave(" = %p", cell);
@@ -358,25 +362,39 @@ int afs_cell_init(struct afs_net *net, const char *rootcell)
 static void afs_update_cell(struct afs_cell *cell)
 {
 	struct afs_vlserver_list *vllist, *old;
-	time64_t now, expiry;
+	unsigned int min_ttl = READ_ONCE(afs_cell_min_ttl);
+	unsigned int max_ttl = READ_ONCE(afs_cell_max_ttl);
+	time64_t now, expiry = 0;
 
 	_enter("%s", cell->name);
 
 	vllist = afs_dns_query(cell, &expiry);
+
+	now = ktime_get_real_seconds();
+	if (min_ttl > max_ttl)
+		max_ttl = min_ttl;
+	if (expiry < now + min_ttl)
+		expiry = now + min_ttl;
+	else if (expiry > now + max_ttl)
+		expiry = now + max_ttl;
+
 	if (IS_ERR(vllist)) {
 		switch (PTR_ERR(vllist)) {
 		case -ENODATA:
-			/* The DNS said that the cell does not exist */
+		case -EDESTADDRREQ:
+			/* The DNS said that the cell does not exist or there
+			 * weren't any addresses to be had.
+			 */
 			set_bit(AFS_CELL_FL_NOT_FOUND, &cell->flags);
 			clear_bit(AFS_CELL_FL_DNS_FAIL, &cell->flags);
-			cell->dns_expiry = ktime_get_real_seconds() + 61;
+			cell->dns_expiry = expiry;
 			break;
 
 		case -EAGAIN:
 		case -ECONNREFUSED:
 		default:
 			set_bit(AFS_CELL_FL_DNS_FAIL, &cell->flags);
-			cell->dns_expiry = ktime_get_real_seconds() + 10;
+			cell->dns_expiry = now + 10;
 			break;
 		}