[PATCH 3/4] nfsd: filecache: change garbage collection list management.

NeilBrown
2025-01-22T03:56:55Z

The nfsd filecache currently uses list_lru for tracking files recently
used in NFSv3 requests which need to be "garbage collected" when they
have becoming idle - unused for 2-4 seconds.

I do not believe list_lru is a good tool for this. It does not allow the
timeout which filecache requires so we have to add a timeout mechanism
which holds the list_lru lock while the whole list is scanned looking for
entries that haven't been recently accessed. When the list is largish
(even a few hundred) this can block new requests which need the lock to
remove a file to access it.

This patch removes the list_lru and instead uses 2 simple linked lists.
When a file is accessed it is removed from whichever list it is on,
then added to the tail of the first list. Every 2 seconds the second
list is moved to the "freeme" list and the first list is moved to the
second list. This avoids any need to walk a list to find old entries.

Previously a file would be unhashed before being moved to the freeme
list. We don't do that any more. The freeme list is much like the
other two lists (recent and older) in that they all hold a reference to
the file and the file is still hashed. When the nfsd thread processes
the freeme list it now uses the new nfsd_file_release_list() which uses
nfsd_file_cond_queue() to unhash and drop the refcount.

We no longer have a precise count of the size of the lru (recent +
older) as we don't know how big "older" is when it is moved to "freeme".
However the shrinker can cope with an approximation. So we keep a
count of number in the lru and when "older" is moved to "freeme" we
divide that count by 2. When we remove anything from the lru we
decrement that counter but ensure it never goes negative. Naturally
when we add to the lru we increase the counter.

For the filecache stats file, which assumes a global lru, we keep a
separate counter which includes all files in all netns in recent or
older or freeme.

We discard the nf_gc linkage in an nfsd_file and only use nf_lru.
We discard NFSD_FILE_REFERENCED.

This patch drops the nfsd_file_gc_removed() trace point. I couldn't
think of useful information to provide.

Signed-off-by: NeilBrown <neilb@suse.de>
---
fs/nfsd/filecache.c | 215 ++++++++++++++++++++++----------------------
fs/nfsd/filecache.h | 4 +-
fs/nfsd/trace.h | 4 -
3 files changed, 108 insertions(+), 115 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 4f39f6632b35..552feba94f09 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -34,7 +34,6 @@
#include <linux/file.h>
#include <linux/pagemap.h>
#include <linux/sched.h>
-#include <linux/list_lru.h>
#include <linux/fsnotify_backend.h>
#include <linux/fsnotify.h>
#include <linux/seq_file.h>
@@ -63,10 +62,13 @@ static DEFINE_PER_CPU(unsigned long, nfsd_file_evictions);

struct nfsd_fcache_disposal {
spinlock_t lock;
- struct list_lru file_lru;
- struct list_head freeme;
+ struct list_head recent; /* have been used in last 0-2 seconds */
+ struct list_head older; /* haven't been used in last 0-2 seconds */
+ struct list_head freeme; /* ready to be discarded */
+ unsigned long num_gc; /* Approximate size of recent plus older */
struct delayed_work filecache_laundrette;
struct shrinker *file_shrinker;
+ struct nfsd_net *nn;
};

static struct kmem_cache *nfsd_file_slab;
@@ -227,7 +229,6 @@ nfsd_file_alloc(struct net *net, struct inode *inode, unsigned char need,

this_cpu_inc(nfsd_file_allocations);
INIT_LIST_HEAD(&nf->nf_lru);
- INIT_LIST_HEAD(&nf->nf_gc);
nf->nf_birthtime = ktime_get();
nf->nf_file = NULL;
nf->nf_cred = get_current_cred();
@@ -332,12 +333,16 @@ static bool nfsd_file_lru_add(struct nfsd_file *nf)
struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
struct nfsd_fcache_disposal *l = nn->fcache_disposal;

- set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
- if (list_lru_add_obj(&l->file_lru, &nf->nf_lru)) {
+ spin_lock(&l->lock);
+ if (list_empty(&nf->nf_lru)) {
+ list_add_tail(&nf->nf_lru, &l->recent);
+ l->num_gc += 1;
atomic_long_inc(&nfsd_lru_total);
trace_nfsd_file_lru_add(nf);
+ spin_unlock(&l->lock);
return true;
}
+ spin_unlock(&l->lock);
return false;
}

@@ -346,11 +351,17 @@ static bool nfsd_file_lru_remove(struct nfsd_file *nf)
struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
struct nfsd_fcache_disposal *l = nn->fcache_disposal;

- if (list_lru_del_obj(&l->file_lru, &nf->nf_lru)) {
+ spin_lock(&l->lock);
+ if (!list_empty(&nf->nf_lru)) {
+ list_del_init(&nf->nf_lru);
atomic_long_dec(&nfsd_lru_total);
+ if (l->num_gc > 0)
+ l->num_gc -= 1;
trace_nfsd_file_lru_del(nf);
+ spin_unlock(&l->lock);
return true;
}
+ spin_unlock(&l->lock);
return false;
}

@@ -440,12 +451,26 @@ nfsd_file_dispose_list(struct list_head *dispose)
struct nfsd_file *nf;

while (!list_empty(dispose)) {
- nf = list_first_entry(dispose, struct nfsd_file, nf_gc);
- list_del_init(&nf->nf_gc);
+ nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
+ list_del_init(&nf->nf_lru);
nfsd_file_free(nf);
}
}

+static void
+nfsd_file_cond_queue(struct nfsd_file *nf, struct list_head *dispose);
+
+static void
+nfsd_file_release_list(struct list_head *dispose)
+{
+ LIST_HEAD(dispose2);
+ struct nfsd_file *nf, *nf2;
+
+ list_for_each_entry_safe(nf, nf2, dispose, nf_lru)
+ nfsd_file_cond_queue(nf, &dispose2);
+ nfsd_file_dispose_list(&dispose2);
+}
+
/**
* nfsd_file_dispose_list_delayed - move list of dead files to net's freeme list
* @dispose: list of nfsd_files to be disposed
@@ -458,12 +483,12 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
{
while(!list_empty(dispose)) {
struct nfsd_file *nf = list_first_entry(dispose,
- struct nfsd_file, nf_gc);
+ struct nfsd_file, nf_lru);
struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
struct nfsd_fcache_disposal *l = nn->fcache_disposal;

spin_lock(&l->lock);
- list_move_tail(&nf->nf_gc, &l->freeme);
+ list_move_tail(&nf->nf_lru, &l->freeme);
spin_unlock(&l->lock);
svc_wake_up(nn->nfsd_serv);
}
@@ -487,88 +512,32 @@ void nfsd_file_net_dispose(struct nfsd_net *nn)
int i;

spin_lock(&l->lock);
- for (i = 0; i < 8 && !list_empty(&l->freeme); i++)
- list_move(l->freeme.next, &dispose);
+ for (i = 0; i < 8 && !list_empty(&l->freeme); i++) {
+ struct nfsd_file *nf = list_first_entry(
+ &l->freeme, struct nfsd_file, nf_lru);
+
+ /*
+ * Don't throw out files that are still
+ * undergoing I/O or that have uncleared errors
+ * pending.
+ */
+ if (nfsd_file_check_writeback(nf)) {
+ trace_nfsd_file_gc_writeback(nf);
+ list_move(&nf->nf_lru, &l->recent);
+ l->num_gc += 1;
+ } else {
+ trace_nfsd_file_gc_disposed(nf);
+ list_move(&nf->nf_lru, &dispose);
+ this_cpu_inc(nfsd_file_evictions);
+ }
+ }
spin_unlock(&l->lock);
if (!list_empty(&l->freeme))
/* Wake up another thread to share the work
* *before* doing any actual disposing.
*/
svc_wake_up(nn->nfsd_serv);
- nfsd_file_dispose_list(&dispose);
- }
-}
-
-/**
- * nfsd_file_lru_cb - Examine an entry on the LRU list
- * @item: LRU entry to examine
- * @lru: controlling LRU
- * @arg: dispose list
- *
- * Return values:
- * %LRU_REMOVED: @item was removed from the LRU
- * %LRU_ROTATE: @item is to be moved to the LRU tail
- * %LRU_SKIP: @item cannot be evicted
- */
-static enum lru_status
-nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru,
- void *arg)
-{
- struct list_head *head = arg;
- struct nfsd_file *nf = list_entry(item, struct nfsd_file, nf_lru);
-
- /* We should only be dealing with GC entries here */
- WARN_ON_ONCE(!test_bit(NFSD_FILE_GC, &nf->nf_flags));
-
- /*
- * Don't throw out files that are still undergoing I/O or
- * that have uncleared errors pending.
- */
- if (nfsd_file_check_writeback(nf)) {
- trace_nfsd_file_gc_writeback(nf);
- return LRU_SKIP;
- }
-
- /* If it was recently added to the list, skip it */
- if (test_and_clear_bit(NFSD_FILE_REFERENCED, &nf->nf_flags)) {
- trace_nfsd_file_gc_referenced(nf);
- return LRU_ROTATE;
- }
-
- /*
- * Put the reference held on behalf of the LRU. If it wasn't the last
- * one, then just remove it from the LRU and ignore it.
- */
- if (!refcount_dec_and_test(&nf->nf_ref)) {
- trace_nfsd_file_gc_in_use(nf);
- list_lru_isolate(lru, &nf->nf_lru);
- return LRU_REMOVED;
- }
-
- /* Refcount went to zero. Unhash it and queue it to the dispose list */
- nfsd_file_unhash(nf);
- list_lru_isolate(lru, &nf->nf_lru);
- list_add(&nf->nf_gc, head);
- this_cpu_inc(nfsd_file_evictions);
- trace_nfsd_file_gc_disposed(nf);
- return LRU_REMOVED;
-}
-
-static void
-nfsd_file_gc(struct nfsd_fcache_disposal *l)
-{
- unsigned long remaining = list_lru_count(&l->file_lru);
- LIST_HEAD(dispose);
- unsigned long ret;
-
- while (remaining > 0) {
- unsigned long num_to_scan = min(remaining, NFSD_FILE_GC_BATCH);
-
- ret = list_lru_walk(&l->file_lru, nfsd_file_lru_cb,
- &dispose, num_to_scan);
- trace_nfsd_file_gc_removed(ret, list_lru_count(&l->file_lru));
- nfsd_file_dispose_list_delayed(&dispose);
- remaining -= num_to_scan;
+ nfsd_file_release_list(&dispose);
}
}

@@ -577,9 +546,20 @@ nfsd_file_gc_worker(struct work_struct *work)
{
struct nfsd_fcache_disposal *l = container_of(
work, struct nfsd_fcache_disposal, filecache_laundrette.work);
- nfsd_file_gc(l);
- if (list_lru_count(&l->file_lru))
+
+ spin_lock(&l->lock);
+ list_splice_init(&l->older, &l->freeme);
+ list_splice_init(&l->recent, &l->older);
+ /* We don't know how many were moved to 'freeme' and don't want
+ * to waste time counting - guess a half.
+ */
+ l->num_gc /= 2;
+ if (!list_empty(&l->freeme))
+ svc_wake_up(l->nn->nfsd_serv);
+ if (!list_empty(&l->older) || !list_empty(&l->recent))
nfsd_file_schedule_laundrette(l);
+ spin_unlock(&l->lock);
+
}

static unsigned long
@@ -587,22 +567,40 @@ nfsd_file_lru_count(struct shrinker *s, struct shrink_control *sc)
{
struct nfsd_fcache_disposal *l = s->private_data;

- return list_lru_count(&l->file_lru);
+ return l->num_gc;
}

static unsigned long
nfsd_file_lru_scan(struct shrinker *s, struct shrink_control *sc)
{
struct nfsd_fcache_disposal *l = s->private_data;
+ struct nfsd_file *nf;
+ int scanned = 0;
+
+ spin_lock(&l->lock);
+ while (scanned < sc->nr_to_scan &&
+ (nf = list_first_entry_or_null(&l->older,
+ struct nfsd_file, nf_lru)) != NULL) {
+ list_del_init(&nf->nf_lru);
+ list_add_tail(&nf->nf_lru, &l->freeme);
+ if (l->num_gc > 0)
+ l->num_gc -= 1;
+ scanned += 1;
+ }
+ if (scanned > 0)
+ svc_wake_up(l->nn->nfsd_serv);

- LIST_HEAD(dispose);
- unsigned long ret;
+ trace_nfsd_file_shrinker_removed(scanned, l->num_gc);

- ret = list_lru_shrink_walk(&l->file_lru, sc,
- nfsd_file_lru_cb, &dispose);
- trace_nfsd_file_shrinker_removed(ret, list_lru_count(&l->file_lru));
- nfsd_file_dispose_list_delayed(&dispose);
- return ret;
+ while (scanned < sc->nr_to_scan &&
+ (nf = list_first_entry_or_null(&l->recent,
+ struct nfsd_file, nf_lru)) != NULL) {
+ list_del_init(&nf->nf_lru);
+ list_add_tail(&nf->nf_lru, &l->older);
+ scanned += 1;
+ }
+ spin_unlock(&l->lock);
+ return scanned;
}

/**
@@ -615,7 +613,6 @@ nfsd_file_lru_scan(struct shrinker *s, struct shrink_control *sc)
*/
static void
nfsd_file_cond_queue(struct nfsd_file *nf, struct list_head *dispose)
- __must_hold(RCU)
{
int decrement = 1;

@@ -633,7 +630,7 @@ nfsd_file_cond_queue(struct nfsd_file *nf, struct list_head *dispose)

/* If refcount goes to 0, then put on the dispose list */
if (refcount_sub_and_test(decrement, &nf->nf_ref)) {
- list_add(&nf->nf_gc, dispose);
+ list_add(&nf->nf_lru, dispose);
trace_nfsd_file_closing(nf);
}
}
@@ -858,7 +855,10 @@ nfsd_alloc_fcache_disposal(void)
return NULL;
spin_lock_init(&l->lock);
INIT_DELAYED_WORK(&l->filecache_laundrette, nfsd_file_gc_worker);
+ INIT_LIST_HEAD(&l->recent);
+ INIT_LIST_HEAD(&l->older);
INIT_LIST_HEAD(&l->freeme);
+ l->num_gc = 0;
l->file_shrinker = shrinker_alloc(0, "nfsd-filecache");
if (!l->file_shrinker) {
pr_err("nfsd: failed to allocate nfsd_file_shrinker\n");
@@ -870,13 +870,6 @@ nfsd_alloc_fcache_disposal(void)
l->file_shrinker->seeks = 1;
l->file_shrinker->private_data = l;

- if (list_lru_init(&l->file_lru)) {
- pr_err("nfsd: failed to init nfsd_file_lru\n");
- shrinker_free(l->file_shrinker);
- kfree(l);
- return NULL;
- }
-
shrinker_register(l->file_shrinker);
return l;
}
@@ -886,8 +879,12 @@ nfsd_free_fcache_disposal(struct nfsd_fcache_disposal *l)
{
cancel_delayed_work_sync(&l->filecache_laundrette);
shrinker_free(l->file_shrinker);
- list_lru_destroy(&l->file_lru);
- nfsd_file_dispose_list(&l->freeme);
+ nfsd_file_release_list(&l->recent);
+ WARN_ON_ONCE(!list_empty(&l->recent));
+ nfsd_file_release_list(&l->older);
+ WARN_ON_ONCE(!list_empty(&l->older));
+ nfsd_file_release_list(&l->freeme);
+ WARN_ON_ONCE(!list_empty(&l->freeme));
kfree(l);
}

@@ -906,6 +903,8 @@ nfsd_file_cache_start_net(struct net *net)
struct nfsd_net *nn = net_generic(net, nfsd_net_id);

nn->fcache_disposal = nfsd_alloc_fcache_disposal();
+ if (nn->fcache_disposal)
+ nn->fcache_disposal->nn = nn;
return nn->fcache_disposal ? 0 : -ENOMEM;
}

diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
index a09a851d510e..02059b6c5e9a 100644
--- a/fs/nfsd/filecache.h
+++ b/fs/nfsd/filecache.h
@@ -42,15 +42,13 @@ struct nfsd_file {
struct net *nf_net;
#define NFSD_FILE_HASHED (0)
#define NFSD_FILE_PENDING (1)
-#define NFSD_FILE_REFERENCED (2)
-#define NFSD_FILE_GC (3)
+#define NFSD_FILE_GC (2)
unsigned long nf_flags;
refcount_t nf_ref;
unsigned char nf_may;

struct nfsd_file_mark *nf_mark;
struct list_head nf_lru;
- struct list_head nf_gc;
struct rcu_head nf_rcu;
ktime_t nf_birthtime;
};
diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
index ad2c0c432d08..efa683541ed5 100644
--- a/fs/nfsd/trace.h
+++ b/fs/nfsd/trace.h
@@ -1038,7 +1038,6 @@ DEFINE_CLID_EVENT(confirmed_r);
__print_flags(val, "|", \
{ 1 << NFSD_FILE_HASHED, "HASHED" }, \
{ 1 << NFSD_FILE_PENDING, "PENDING" }, \
- { 1 << NFSD_FILE_REFERENCED, "REFERENCED" }, \
{ 1 << NFSD_FILE_GC, "GC" })

DECLARE_EVENT_CLASS(nfsd_file_class,
@@ -1314,9 +1313,7 @@ DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_lru_add);
DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_lru_add_disposed);
DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_lru_del);
DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_lru_del_disposed);
-DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_in_use);
DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_writeback);
-DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_referenced);
DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_disposed);

DECLARE_EVENT_CLASS(nfsd_file_lruwalk_class,
@@ -1345,7 +1342,6 @@ DEFINE_EVENT(nfsd_file_lruwalk_class, name, \
), \
TP_ARGS(removed, remaining))

-DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_gc_removed);
DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_shrinker_removed);

TRACE_EVENT(nfsd_file_close,
--
2.47.1