Blob Blame History Raw
From: NeilBrown <neilb@suse.de>
Date: Fri, 20 Sep 2019 16:33:16 +1000
Subject: [PATCH] nfsd: handle drc over-allocation gracefully.
Git-commit: 7f49fd5d7acd82163685559045d04d49433581cc
Patch-mainline: v5.4-rc1
References: bsc#1150381

Currently, if there are more clients than allowed for by the
space allocation in set_max_drc(), we fail a SESSION_CREATE
request with NFS4ERR_DELAY.
This means that the client retries indefinitely, which isn't
a user-friendly response.

The RFC requires NFS4ERR_NOSPC, but that would at best result in a
clean failure on the client, which is not much more friendly.

The current space allocation is a best-guess and doesn't provide any
guarantees, we could still run out of space when trying to allocate
drc space.

So fail more gracefully - always give out at least one slot.
If all clients used all the space in all slots, we might start getting
memory pressure, but that is possible anyway.

So ensure 'num' is always at least 1, and remove the test for it
being zero.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Acked-by: NeilBrown <neilb@suse.com>

---
 fs/nfsd/nfs4state.c |   19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1505,14 +1505,25 @@ static u32 nfsd4_get_drc_mem(struct nfsd
 	unsigned long avail, total_avail;
 
 	spin_lock(&nfsd_drc_lock);
-	total_avail = nfsd_drc_max_mem - nfsd_drc_mem_used;
+	if (nfsd_drc_max_mem > nfsd_drc_mem_used)
+		total_avail = nfsd_drc_max_mem - nfsd_drc_mem_used;
+	else
+		/* We have handed out more space than we chose in
+		 * set_max_drc() to allow.  That isn't really a
+		 * problem as long as that doesn't make us think we
+		 * have lots more due to integer overflow.
+		 */
+		total_avail = 0;
 	avail = min((unsigned long)NFSD_MAX_MEM_PER_SESSION, total_avail);
 	/*
 	 * Never use more than a third of the remaining memory,
-	 * unless it's the only way to give this client a slot:
+	 * unless it's the only way to give this client a slot.
+	 * Give the client one slot even if that would require
+	 * over-allocation--it is better than failure.
 	 */
 	avail = clamp_t(unsigned long, avail, slotsize, total_avail/3);
 	num = min_t(int, num, avail / slotsize);
+	num = max_t(int, num, 1);
 	nfsd_drc_mem_used += num * slotsize;
 	spin_unlock(&nfsd_drc_lock);
 
@@ -2702,10 +2713,10 @@ static __be32 check_forechannel_attrs(st
 	 * performance.  When short on memory we therefore prefer to
 	 * decrease number of slots instead of their size.  Clients that
 	 * request larger slots than they need will get poor results:
+	 * Note that we always allow at least one slot, because our
+	 * accounting is soft and provides no guarantees either way.
 	 */
 	ca->maxreqs = nfsd4_get_drc_mem(ca);
-	if (!ca->maxreqs)
-		return nfserr_jukebox;
 
 	return nfs_ok;
 }