NFSD: Don't hand out delegations for 30 seconds after recalling them.

neilbrown · J. Bruce Fields · commit 6282cd565553 · 2014-06-17T16:42:47.000-04:00
If nfsd needs to recall a delegation for some reason it implies that there is
contention on the file, so further delegations should not be handed out.

The current code fails to do so, and the result is effectively a
live-lock under some workloads: a client attempting a conflicting
operation on a read-delegated file receives NFS4ERR_DELAY and retries
the operation, but by the time it retries the server may already have
given out another delegation.

We could simply avoid delegations for (say) 30 seconds after any recall, but
this is probably too heavy handed.

We could keep a list of inodes (or inode numbers or filehandles) for recalled
delegations, but that requires memory allocation and searching.

The approach taken here is to use a bloom filter to record the filehandles
which are currently blocked from delegation, and to accept the cost of a few
false positives.

We have 2 bloom filters, each of which is valid for 30 seconds.   When a
delegation is recalled the filehandle is added to one filter and will remain
disabled for between 30 and 60 seconds.

We keep a count of the number of filehandles that have been added, so when
that count is zero we can bypass all other tests.

The bloom filters have 256 bits and 3 hash functions.  This should allow a
couple of dozen blocked  filehandles with minimal false positives.  If many
more filehandles are all blocked at once, behaviour will degrade towards
rejecting all delegations for between 30 and 60 seconds, then resetting and
allowing new delegations.

Signed-off-by: NeilBrown &lt;neilb@suse.de&gt;
Signed-off-by: J. Bruce Fields &lt;bfields@redhat.com&gt;
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
@@ -41,6 +41,7 @@
 #include <linux/ratelimit.h>
 #include <linux/sunrpc/svcauth_gss.h>
 #include <linux/sunrpc/addr.h>
+#include <linux/hash.h>
 #include "xdr4.h"
 #include "xdr4cb.h"
 #include "vfs.h"
@@ -364,6 +365,79 @@ static struct nfs4_ol_stateid * nfs4_alloc_stateid(struct nfs4_client *clp)
 	return openlockstateid(nfs4_alloc_stid(clp, stateid_slab));
 }
 
+/*
+ * When we recall a delegation, we should be careful not to hand it
+ * out again straight away.
+ * To ensure this we keep a pair of bloom filters ('new' and 'old')
+ * in which the filehandles of recalled delegations are "stored".
+ * If a filehandle appear in either filter, a delegation is blocked.
+ * When a delegation is recalled, the filehandle is stored in the "new"
+ * filter.
+ * Every 30 seconds we swap the filters and clear the "new" one,
+ * unless both are empty of course.
+ *
+ * Each filter is 256 bits.  We hash the filehandle to 32bit and use the
+ * low 3 bytes as hash-table indices.
+ *
+ * 'state_lock', which is always held when block_delegations() is called,
+ * is used to manage concurrent access.  Testing does not need the lock
+ * except when swapping the two filters.
+ */
+static struct bloom_pair {
+	int	entries, old_entries;
+	time_t	swap_time;
+	int	new; /* index into 'set' */
+	DECLARE_BITMAP(set[2], 256);
+} blocked_delegations;
+
+static int delegation_blocked(struct knfsd_fh *fh)
+{
+	u32 hash;
+	struct bloom_pair *bd = &blocked_delegations;
+
+	if (bd->entries == 0)
+		return 0;
+	if (seconds_since_boot() - bd->swap_time > 30) {
+		spin_lock(&state_lock);
+		if (seconds_since_boot() - bd->swap_time > 30) {
+			bd->entries -= bd->old_entries;
+			bd->old_entries = bd->entries;
+			memset(bd->set[bd->new], 0,
+			       sizeof(bd->set[0]));
+			bd->new = 1-bd->new;
+			bd->swap_time = seconds_since_boot();
+		}
+		spin_unlock(&state_lock);
+	}
+	hash = arch_fast_hash(&fh->fh_base, fh->fh_size, 0);
+	if (test_bit(hash&255, bd->set[0]) &&
+	    test_bit((hash>>8)&255, bd->set[0]) &&
+	    test_bit((hash>>16)&255, bd->set[0]))
+		return 1;
+
+	if (test_bit(hash&255, bd->set[1]) &&
+	    test_bit((hash>>8)&255, bd->set[1]) &&
+	    test_bit((hash>>16)&255, bd->set[1]))
+		return 1;
+
+	return 0;
+}
+
+static void block_delegations(struct knfsd_fh *fh)
+{
+	u32 hash;
+	struct bloom_pair *bd = &blocked_delegations;
+
+	hash = arch_fast_hash(&fh->fh_base, fh->fh_size, 0);
+
+	__set_bit(hash&255, bd->set[bd->new]);
+	__set_bit((hash>>8)&255, bd->set[bd->new]);
+	__set_bit((hash>>16)&255, bd->set[bd->new]);
+	if (bd->entries == 0)
+		bd->swap_time = seconds_since_boot();
+	bd->entries += 1;
+}
+
 static struct nfs4_delegation *
 alloc_init_deleg(struct nfs4_client *clp, struct nfs4_ol_stateid *stp, struct svc_fh *current_fh)
 {
@@ -372,6 +446,8 @@ alloc_init_deleg(struct nfs4_client *clp, struct nfs4_ol_stateid *stp, struct sv
 	dprintk("NFSD alloc_init_deleg\n");
 	if (num_delegations > max_delegations)
 		return NULL;
+	if (delegation_blocked(&current_fh->fh_handle))
+		return NULL;
 	dp = delegstateid(nfs4_alloc_stid(clp, deleg_slab));
 	if (dp == NULL)
 		return dp;
@@ -2770,6 +2846,8 @@ static void nfsd_break_one_deleg(struct nfs4_delegation *dp)
 	/* Only place dl_time is set; protected by i_lock: */
 	dp->dl_time = get_seconds();
 
+	block_delegations(&dp->dl_fh);
+
 	nfsd4_cb_recall(dp);
 }
 
