cleancache: remove limit on the number of cleancache enabled filesystems

The limit equals 32 and is imposed by the number of entries in the
fs_poolid_map and shared_fs_poolid_map.  Nowadays it is insufficient,
because with containers on board a Linux host can have hundreds of
active fs mounts.

These maps were introduced by commit 49a9ab815acb8 ("mm: cleancache:
lazy initialization to allow tmem backends to build/run as modules") in
order to allow compiling cleancache drivers as modules.  Real pool ids
are stored in these maps while super_block->cleancache_poolid points to
an entry in the map, so that on cleancache registration we can walk over
all (if there are <= 32 of them, of course) cleancache-enabled super
blocks and assign real pool ids.

Actually, there is absolutely no need in these maps, because we can
iterate over all super blocks immediately using iterate_supers.  This is
not racy, because cleancache_init_ops is called from mount_fs with
super_block->s_umount held for writing, while iterate_supers takes this
semaphore for reading, so if we call iterate_supers after setting
cleancache_ops, all super blocks that had been created before
cleancache_register_ops was called will be assigned pool ids by the
action function of iterate_supers while all newer super blocks will
receive it in cleancache_init_fs.

This patch therefore removes the maps and hence the artificial limit on
the number of cleancache enabled filesystems.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Stefan Hengelein <ilendir@googlemail.com>
Cc: Florian Schmaus <fschmaus@gmail.com>
Cc: Andor Daam <andor.daam@googlemail.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 94 insertions, 182 deletions

diff --git a/fs/super.c b/fs/super.c
index 2b7dc90ccdbb..928c20f47af9 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -224,7 +224,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
         s->s_maxbytes = MAX_NON_LFS;
         s->s_op = &default_op;
         s->s_time_gran = 1000000000;
-        s->cleancache_poolid = -1;
+        s->cleancache_poolid = CLEANCACHE_NO_POOL;
 
         s->s_shrink.seeks = DEFAULT_SEEKS;
         s->s_shrink.scan_objects = super_cache_scan;
diff --git a/include/linux/cleancache.h b/include/linux/cleancache.h
index b23611f43cfb..bda5ec0b4b4d 100644
--- a/include/linux/cleancache.h
+++ b/include/linux/cleancache.h
@@ -5,6 +5,10 @@
 #include <linux/exportfs.h>
 #include <linux/mm.h>
 
+#define CLEANCACHE_NO_POOL              -1
+#define CLEANCACHE_NO_BACKEND           -2
+#define CLEANCACHE_NO_BACKEND_SHARED    -3
+
 #define CLEANCACHE_KEY_MAX 6
 
 /*
diff --git a/mm/cleancache.c b/mm/cleancache.c
index aa10f9a3bc88..8fc50811119b 100644
--- a/mm/cleancache.c
+++ b/mm/cleancache.c
@@ -19,7 +19,7 @@
 #include <linux/cleancache.h>
 
 /*
- * cleancache_ops is set by cleancache_ops_register to contain the pointers
+ * cleancache_ops is set by cleancache_register_ops to contain the pointers
  * to the cleancache "backend" implementation functions.
  */
 static struct cleancache_ops *cleancache_ops __read_mostly;
@@ -34,104 +34,78 @@ static u64 cleancache_failed_gets;
 static u64 cleancache_puts;
 static u64 cleancache_invalidates;
 
-/*
- * When no backend is registered all calls to init_fs and init_shared_fs
- * are registered and fake poolids (FAKE_FS_POOLID_OFFSET or
- * FAKE_SHARED_FS_POOLID_OFFSET, plus offset in the respective array
- * [shared_|]fs_poolid_map) are given to the respective super block
- * (sb->cleancache_poolid) and no tmem_pools are created. When a backend
- * registers with cleancache the previous calls to init_fs and init_shared_fs
- * are executed to create tmem_pools and set the respective poolids. While no
- * backend is registered all "puts", "gets" and "flushes" are ignored or failed.
- */
-#define MAX_INITIALIZABLE_FS 32
-#define FAKE_FS_POOLID_OFFSET 1000
-#define FAKE_SHARED_FS_POOLID_OFFSET 2000
-
-#define FS_NO_BACKEND (-1)
-#define FS_UNKNOWN (-2)
-static int fs_poolid_map[MAX_INITIALIZABLE_FS];
-static int shared_fs_poolid_map[MAX_INITIALIZABLE_FS];
-static char *uuids[MAX_INITIALIZABLE_FS];
-/*
- * Mutex for the [shared_|]fs_poolid_map to guard against multiple threads
- * invoking umount (and ending in __cleancache_invalidate_fs) and also multiple
- * threads calling mount (and ending up in __cleancache_init_[shared|]fs).
- */
-static DEFINE_MUTEX(poolid_mutex);
-/*
- * When set to false (default) all calls to the cleancache functions, except
- * the __cleancache_invalidate_fs and __cleancache_init_[shared|]fs are guarded
- * by the if (!cleancache_ops) return. This means multiple threads (from
- * different filesystems) will be checking cleancache_ops. The usage of a
- * bool instead of a atomic_t or a bool guarded by a spinlock is OK - we are
- * OK if the time between the backend's have been initialized (and
- * cleancache_ops has been set to not NULL) and when the filesystems start
- * actually calling the backends. The inverse (when unloading) is obviously
- * not good - but this shim does not do that (yet).
- */
-
-/*
- * The backends and filesystems work all asynchronously. This is b/c the
- * backends can be built as modules.
- * The usual sequence of events is:
- *      a) mount /      -> __cleancache_init_fs is called. We set the
- *              [shared_|]fs_poolid_map and uuids for.
- *
- *      b). user does I/Os -> we call the rest of __cleancache_* functions
- *              which return immediately as cleancache_ops is false.
- *
- *      c). modprobe zcache -> cleancache_register_ops. We init the backend
- *              and set cleancache_ops to true, and for any fs_poolid_map
- *              (which is set by __cleancache_init_fs) we initialize the poolid.
- *
- *      d). user does I/Os -> now that cleancache_ops is true all the
- *              __cleancache_* functions can call the backend. They all check
- *              that fs_poolid_map is valid and if so invoke the backend.
- *
- *      e). umount /    -> __cleancache_invalidate_fs, the fs_poolid_map is
- *              reset (which is the second check in the __cleancache_* ops
- *              to call the backend).
- *
- * The sequence of event could also be c), followed by a), and d). and e). The
- * c) would not happen anymore. There is also the chance of c), and one thread
- * doing a) + d), and another doing e). For that case we depend on the
- * filesystem calling __cleancache_invalidate_fs in the proper sequence (so
- * that it handles all I/Os before it invalidates the fs (which is last part
- * of unmounting process).
- *
- * Note: The acute reader will notice that there is no "rmmod zcache" case.
- * This is b/c the functionality for that is not yet implemented and when
- * done, will require some extra locking not yet devised.
- */
+static void cleancache_register_ops_sb(struct super_block *sb, void *unused)
+{
+        switch (sb->cleancache_poolid) {
+        case CLEANCACHE_NO_BACKEND:
+                __cleancache_init_fs(sb);
+                break;
+        case CLEANCACHE_NO_BACKEND_SHARED:
+                __cleancache_init_shared_fs(sb);
+                break;
+        }
+}
 
 /*
  * Register operations for cleancache. Returns 0 on success.
  */
 int cleancache_register_ops(struct cleancache_ops *ops)
 {
-        int i;
-
-        mutex_lock(&poolid_mutex);
-        if (cleancache_ops) {
-                mutex_unlock(&poolid_mutex);
+        if (cmpxchg(&cleancache_ops, NULL, ops))
                 return -EBUSY;
-        }
-        for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
-                if (fs_poolid_map[i] == FS_NO_BACKEND)
-                        fs_poolid_map[i] = ops->init_fs(PAGE_SIZE);
-                if (shared_fs_poolid_map[i] == FS_NO_BACKEND)
-                        shared_fs_poolid_map[i] = ops->init_shared_fs
-                                        (uuids[i], PAGE_SIZE);
-        }
+
         /*
-         * We MUST set cleancache_ops _after_ we have called the backends
-         * init_fs or init_shared_fs functions. Otherwise the compiler might
-         * re-order where cleancache_ops is set in this function.
+         * A cleancache backend can be built as a module and hence loaded after
+         * a cleancache enabled filesystem has called cleancache_init_fs. To
+         * handle such a scenario, here we call ->init_fs or ->init_shared_fs
+         * for each active super block. To differentiate between local and
+         * shared filesystems, we temporarily initialize sb->cleancache_poolid
+         * to CLEANCACHE_NO_BACKEND or CLEANCACHE_NO_BACKEND_SHARED
+         * respectively in case there is no backend registered at the time
+         * cleancache_init_fs or cleancache_init_shared_fs is called.
+         *
+         * Since filesystems can be mounted concurrently with cleancache
+         * backend registration, we have to be careful to guarantee that all
+         * cleancache enabled filesystems that has been mounted by the time
+         * cleancache_register_ops is called has got and all mounted later will
+         * get cleancache_poolid. This is assured by the following statements
+         * tied together:
+         *
+         * a) iterate_supers skips only those super blocks that has started
+         *    ->kill_sb
+         *
+         * b) if iterate_supers encounters a super block that has not finished
+         *    ->mount yet, it waits until it is finished
+         *
+         * c) cleancache_init_fs is called from ->mount and
+         *    cleancache_invalidate_fs is called from ->kill_sb
+         *
+         * d) we call iterate_supers after cleancache_ops has been set
+         *
+         * From a) it follows that if iterate_supers skips a super block, then
+         * either the super block is already dead, in which case we do not need
+         * to bother initializing cleancache for it, or it was mounted after we
+         * initiated iterate_supers. In the latter case, it must have seen
+         * cleancache_ops set according to d) and initialized cleancache from
+         * ->mount by itself according to c). This proves that we call
+         * ->init_fs at least once for each active super block.
+         *
+         * From b) and c) it follows that if iterate_supers encounters a super
+         * block that has already started ->init_fs, it will wait until ->mount
+         * and hence ->init_fs has finished, then check cleancache_poolid, see
+         * that it has already been set and therefore do nothing. This proves
+         * that we call ->init_fs no more than once for each super block.
+         *
+         * Combined together, the last two paragraphs prove the function
+         * correctness.
+         *
+         * Note that various cleancache callbacks may proceed before this
+         * function is called or even concurrently with it, but since
+         * CLEANCACHE_NO_BACKEND is negative, they will all result in a noop
+         * until the corresponding ->init_fs has been actually called and
+         * cleancache_ops has been set.
          */
-        barrier();
-        cleancache_ops = ops;
-        mutex_unlock(&poolid_mutex);
+        iterate_supers(cleancache_register_ops_sb, NULL);
         return 0;
 }
 EXPORT_SYMBOL(cleancache_register_ops);
@@ -139,42 +113,28 @@ EXPORT_SYMBOL(cleancache_register_ops);
 /* Called by a cleancache-enabled filesystem at time of mount */
 void __cleancache_init_fs(struct super_block *sb)
 {
-        int i;
+        int pool_id = CLEANCACHE_NO_BACKEND;
 
-        mutex_lock(&poolid_mutex);
-        for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
-                if (fs_poolid_map[i] == FS_UNKNOWN) {
-                        sb->cleancache_poolid = i + FAKE_FS_POOLID_OFFSET;
-                        if (cleancache_ops)
-                                fs_poolid_map[i] = cleancache_ops->init_fs(PAGE_SIZE);
-                        else
-                                fs_poolid_map[i] = FS_NO_BACKEND;
-                        break;
-                }
+        if (cleancache_ops) {
+                pool_id = cleancache_ops->init_fs(PAGE_SIZE);
+                if (pool_id < 0)
+                        pool_id = CLEANCACHE_NO_POOL;
         }
-        mutex_unlock(&poolid_mutex);
+        sb->cleancache_poolid = pool_id;
 }
 EXPORT_SYMBOL(__cleancache_init_fs);
 
 /* Called by a cleancache-enabled clustered filesystem at time of mount */
 void __cleancache_init_shared_fs(struct super_block *sb)
 {
-        int i;
+        int pool_id = CLEANCACHE_NO_BACKEND_SHARED;
 
-        mutex_lock(&poolid_mutex);
-        for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
-                if (shared_fs_poolid_map[i] == FS_UNKNOWN) {
-                        sb->cleancache_poolid = i + FAKE_SHARED_FS_POOLID_OFFSET;
-                        uuids[i] = sb->s_uuid;
-                        if (cleancache_ops)
-                                shared_fs_poolid_map[i] = cleancache_ops->init_shared_fs
-                                                (sb->s_uuid, PAGE_SIZE);
-                        else
-                                shared_fs_poolid_map[i] = FS_NO_BACKEND;
-                        break;
-                }
+        if (cleancache_ops) {
+                pool_id = cleancache_ops->init_shared_fs(sb->s_uuid, PAGE_SIZE);
+                if (pool_id < 0)
+                        pool_id = CLEANCACHE_NO_POOL;
         }
-        mutex_unlock(&poolid_mutex);
+        sb->cleancache_poolid = pool_id;
 }
 EXPORT_SYMBOL(__cleancache_init_shared_fs);
 
@@ -204,19 +164,6 @@ static int cleancache_get_key(struct inode *inode,
 }
 
 /*
- * Returns a pool_id that is associated with a given fake poolid.
- */
-static int get_poolid_from_fake(int fake_pool_id)
-{
-        if (fake_pool_id >= FAKE_SHARED_FS_POOLID_OFFSET)
-                return shared_fs_poolid_map[fake_pool_id -
-                        FAKE_SHARED_FS_POOLID_OFFSET];
-        else if (fake_pool_id >= FAKE_FS_POOLID_OFFSET)
-                return fs_poolid_map[fake_pool_id - FAKE_FS_POOLID_OFFSET];
-        return FS_NO_BACKEND;
-}
-
-/*
  * "Get" data from cleancache associated with the poolid/inode/index
  * that were specified when the data was put to cleanache and, if
  * successful, use it to fill the specified page with data and return 0.
@@ -231,7 +178,6 @@ int __cleancache_get_page(struct page *page)
 {
         int ret = -1;
         int pool_id;
-        int fake_pool_id;
         struct cleancache_filekey key = { .u.key = { 0 } };
 
         if (!cleancache_ops) {
@@ -240,17 +186,14 @@ int __cleancache_get_page(struct page *page)
         }
 
         VM_BUG_ON_PAGE(!PageLocked(page), page);
-        fake_pool_id = page->mapping->host->i_sb->cleancache_poolid;
-        if (fake_pool_id < 0)
+        pool_id = page->mapping->host->i_sb->cleancache_poolid;
+        if (pool_id < 0)
                 goto out;
-        pool_id = get_poolid_from_fake(fake_pool_id);
 
         if (cleancache_get_key(page->mapping->host, &key) < 0)
                 goto out;
 
-        if (pool_id >= 0)
-                ret = cleancache_ops->get_page(pool_id,
-                                key, page->index, page);
+        ret = cleancache_ops->get_page(pool_id, key, page->index, page);
         if (ret == 0)
                 cleancache_succ_gets++;
         else
@@ -273,7 +216,6 @@ EXPORT_SYMBOL(__cleancache_get_page);
 void __cleancache_put_page(struct page *page)
 {
         int pool_id;
-        int fake_pool_id;
         struct cleancache_filekey key = { .u.key = { 0 } };
 
         if (!cleancache_ops) {
@@ -282,12 +224,7 @@ void __cleancache_put_page(struct page *page)
         }
 
         VM_BUG_ON_PAGE(!PageLocked(page), page);
-        fake_pool_id = page->mapping->host->i_sb->cleancache_poolid;
-        if (fake_pool_id < 0)
-                return;
-
-        pool_id = get_poolid_from_fake(fake_pool_id);
-
+        pool_id = page->mapping->host->i_sb->cleancache_poolid;
         if (pool_id >= 0 &&
                 cleancache_get_key(page->mapping->host, &key) >= 0) {
                 cleancache_ops->put_page(pool_id, key, page->index, page);
@@ -308,18 +245,13 @@ void __cleancache_invalidate_page(struct address_space *mapping,
                                         struct page *page)
 {
         /* careful... page->mapping is NULL sometimes when this is called */
-        int pool_id;
-        int fake_pool_id = mapping->host->i_sb->cleancache_poolid;
+        int pool_id = mapping->host->i_sb->cleancache_poolid;
         struct cleancache_filekey key = { .u.key = { 0 } };
 
         if (!cleancache_ops)
                 return;
 
-        if (fake_pool_id >= 0) {
-                pool_id = get_poolid_from_fake(fake_pool_id);
-                if (pool_id < 0)
-                        return;
-
+        if (pool_id >= 0) {
                 VM_BUG_ON_PAGE(!PageLocked(page), page);
                 if (cleancache_get_key(mapping->host, &key) >= 0) {
                         cleancache_ops->invalidate_page(pool_id,
@@ -341,18 +273,12 @@ EXPORT_SYMBOL(__cleancache_invalidate_page);
  */
 void __cleancache_invalidate_inode(struct address_space *mapping)
 {
-        int pool_id;
-        int fake_pool_id = mapping->host->i_sb->cleancache_poolid;
+        int pool_id = mapping->host->i_sb->cleancache_poolid;
         struct cleancache_filekey key = { .u.key = { 0 } };
 
         if (!cleancache_ops)
                 return;
 
-        if (fake_pool_id < 0)
-                return;
-
-        pool_id = get_poolid_from_fake(fake_pool_id);
-
         if (pool_id >= 0 && cleancache_get_key(mapping->host, &key) >= 0)
                 cleancache_ops->invalidate_inode(pool_id, key);
 }
@@ -365,32 +291,18 @@ EXPORT_SYMBOL(__cleancache_invalidate_inode);
  */
 void __cleancache_invalidate_fs(struct super_block *sb)
 {
-        int index;
-        int fake_pool_id = sb->cleancache_poolid;
-        int old_poolid = fake_pool_id;
+        int pool_id;
 
-        mutex_lock(&poolid_mutex);
-        if (fake_pool_id >= FAKE_SHARED_FS_POOLID_OFFSET) {
-                index = fake_pool_id - FAKE_SHARED_FS_POOLID_OFFSET;
-                old_poolid = shared_fs_poolid_map[index];
-                shared_fs_poolid_map[index] = FS_UNKNOWN;
-                uuids[index] = NULL;
-        } else if (fake_pool_id >= FAKE_FS_POOLID_OFFSET) {
-                index = fake_pool_id - FAKE_FS_POOLID_OFFSET;
-                old_poolid = fs_poolid_map[index];
-                fs_poolid_map[index] = FS_UNKNOWN;
-        }
-        sb->cleancache_poolid = -1;
-        if (cleancache_ops)
-                cleancache_ops->invalidate_fs(old_poolid);
-        mutex_unlock(&poolid_mutex);
+        pool_id = sb->cleancache_poolid;
+        sb->cleancache_poolid = CLEANCACHE_NO_POOL;
+
+        if (cleancache_ops && pool_id >= 0)
+                cleancache_ops->invalidate_fs(pool_id);
 }
 EXPORT_SYMBOL(__cleancache_invalidate_fs);
 
 static int __init init_cleancache(void)
 {
-        int i;
-
 #ifdef CONFIG_DEBUG_FS
         struct dentry *root = debugfs_create_dir("cleancache", NULL);
         if (root == NULL)
@@ -402,10 +314,6 @@ static int __init init_cleancache(void)
         debugfs_create_u64("invalidates", S_IRUGO,
                                 root, &cleancache_invalidates);
 #endif
-        for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
-                fs_poolid_map[i] = FS_UNKNOWN;
-                shared_fs_poolid_map[i] = FS_UNKNOWN;
-        }
         return 0;
 }
 module_init(init_cleancache)