Date:	Sun, 03 Feb 2013 02:06:30 -0600
From:	Ric Mason <ric.masonn@...il.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
Cc:	dan.magenheimer@...cle.com, konrad.wilk@...cle.com,
	sjenning@...ux.vnet.ibm.com, gregkh@...uxfoundation.org,
	akpm@...ux-foundation.org, ngupta@...are.org,
	rcj@...ux.vnet.ibm.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	Stefan Hengelein <ilendir@...glemail.com>,
	Florian Schmaus <fschmaus@...il.com>,
	Andor Daam <andor.daam@...glemail.com>
Subject: Re: [PATCH 01/15] mm: cleancache: lazy initialization to allow tmem
 backends to build/run as modules

Hi Konrad,
On Fri, 2013-02-01 at 15:22 -0500, Konrad Rzeszutek Wilk wrote:
> From: Dan Magenheimer <dan.magenheimer@...cle.com>
> 
> With the goal of allowing tmem backends (zcache, ramster, Xen tmem) to be
> built/loaded as modules rather than built-in and enabled by a boot parameter,

Which boot parameter? I can't find it in
Documentation/kernl-parameters.txt

> this patch provides "lazy initialization", allowing backends to register to
> cleancache even after filesystems were mounted. Calls to init_fs and
> init_shared_fs are remembered as fake poolids but no real tmem_pools created.
> On backend registration the fake poolids are mapped to real poolids and
> respective tmem_pools.
> 
> Signed-off-by: Stefan Hengelein <ilendir@...glemail.com>
> Signed-off-by: Florian Schmaus <fschmaus@...il.com>
> Signed-off-by: Andor Daam <andor.daam@...glemail.com>
> Signed-off-by: Dan Magenheimer <dan.magenheimer@...cle.com>
> [v1: Minor fixes: used #define for some values and bools]
> [v2: Removed CLEANCACHE_HAS_LAZY_INIT]
> [v3: Added more comments, added a lock for [shared_|]fs_poolid_map]
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  mm/cleancache.c | 240 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 219 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/cleancache.c b/mm/cleancache.c
> index 32e6f41..e4dc314 100644
> --- a/mm/cleancache.c
> +++ b/mm/cleancache.c
> @@ -45,15 +45,99 @@ static u64 cleancache_puts;
>  static u64 cleancache_invalidates;
>  
>  /*
> - * register operations for cleancache, returning previous thus allowing
> - * detection of multiple backends and possible nesting
> + * 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 (!backend_registered) return. This means multiple threads (from
> + * different filesystems) will be checking backend_registered. 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
> + * backend_registered has been set to true) 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).
> + */
> +static bool backend_registered __read_mostly;
> +
> +/*
> + * The backends and filesystems work all asynchronously. This is b/c the

What's the meaning of b/c?zx

> + * 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 backend_registered is false.
> + *
> + * 	c). modprobe zcache -> cleancache_register_ops. We init the backend
> + * 		and set backend_registered 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 backend_registered 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.
> + */
> +
> +/*
> + * Register operations for cleancache, returning previous thus allowing
> + * detection of multiple backends and possible nesting.
>   */
>  struct cleancache_ops cleancache_register_ops(struct cleancache_ops *ops)
>  {
>  	struct cleancache_ops old = cleancache_ops;
> +	int i;
>  
> +	mutex_lock(&poolid_mutex);
>  	cleancache_ops = *ops;
> -	cleancache_enabled = 1;
> +
> +	backend_registered = true;
> +	for (i = 0; i < MAX_INITIALIZABLE_FS; i++) {
> +		if (fs_poolid_map[i] == FS_NO_BACKEND)
> +			fs_poolid_map[i] = (*cleancache_ops.init_fs)(PAGE_SIZE);
> +		if (shared_fs_poolid_map[i] == FS_NO_BACKEND)
> +			shared_fs_poolid_map[i] = (*cleancache_ops.init_shared_fs)
> +					(uuids[i], PAGE_SIZE);
> +	}
> +out:
> +	mutex_unlock(&poolid_mutex);
>  	return old;
>  }
>  EXPORT_SYMBOL(cleancache_register_ops);
> @@ -61,15 +145,42 @@ EXPORT_SYMBOL(cleancache_register_ops);
>  /* Called by a cleancache-enabled filesystem at time of mount */
>  void __cleancache_init_fs(struct super_block *sb)
>  {
> -	sb->cleancache_poolid = (*cleancache_ops.init_fs)(PAGE_SIZE);
> +	int i;
> +
> +	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 (backend_registered)
> +				fs_poolid_map[i] = (*cleancache_ops.init_fs)(PAGE_SIZE);
> +			else
> +				fs_poolid_map[i] = FS_NO_BACKEND;
> +			break;
> +		}
> +	}
> +	mutex_unlock(&poolid_mutex);
>  }
>  EXPORT_SYMBOL(__cleancache_init_fs);
>  
>  /* Called by a cleancache-enabled clustered filesystem at time of mount */
>  void __cleancache_init_shared_fs(char *uuid, struct super_block *sb)
>  {
> -	sb->cleancache_poolid =
> -		(*cleancache_ops.init_shared_fs)(uuid, PAGE_SIZE);
> +	int i;
> +
> +	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] = uuid;
> +			if (backend_registered)
> +				shared_fs_poolid_map[i] = (*cleancache_ops.init_shared_fs)
> +						(uuid, PAGE_SIZE);
> +			else
> +				shared_fs_poolid_map[i] = FS_NO_BACKEND;
> +			break;
> +		}
> +	}
> +	mutex_unlock(&poolid_mutex);
>  }
>  EXPORT_SYMBOL(__cleancache_init_shared_fs);
>  
> @@ -99,27 +210,53 @@ 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.
>   * The pageframe is unchanged and returns -1 if the get fails.
>   * Page must be locked by caller.
> + *
> + * The function has two checks before any action is taken - whether
> + * a backend is registered and whether the sb->cleancache_poolid
> + * is correct.
>   */
>  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 (!backend_registered) {
> +		cleancache_failed_gets++;
> +		goto out;
> +	}
> +
>  	VM_BUG_ON(!PageLocked(page));
> -	pool_id = page->mapping->host->i_sb->cleancache_poolid;
> -	if (pool_id < 0)
> +	fake_pool_id = page->mapping->host->i_sb->cleancache_poolid;
> +	if (fake_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;
>  
> -	ret = (*cleancache_ops.get_page)(pool_id, key, page->index, page);
> +	if (pool_id >= 0)
> +		ret = (*cleancache_ops.get_page)(pool_id,
> +				key, page->index, page);
>  	if (ret == 0)
>  		cleancache_succ_gets++;
>  	else
> @@ -134,16 +271,31 @@ EXPORT_SYMBOL(__cleancache_get_page);
>   * (previously-obtained per-filesystem) poolid and the page's,
>   * inode and page index.  Page must be locked.  Note that a put_page
>   * always "succeeds", though a subsequent get_page may succeed or fail.
> + *
> + * The function has two checks before any action is taken - whether
> + * a backend is registered and whether the sb->cleancache_poolid
> + * is correct.
>   */
>  void __cleancache_put_page(struct page *page)
>  {
>  	int pool_id;
> +	int fake_pool_id;
>  	struct cleancache_filekey key = { .u.key = { 0 } };
>  
> +	if (!backend_registered) {
> +		cleancache_puts++;
> +		return;
> +	}
> +
>  	VM_BUG_ON(!PageLocked(page));
> -	pool_id = page->mapping->host->i_sb->cleancache_poolid;
> +	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);
> +
>  	if (pool_id >= 0 &&
> -	      cleancache_get_key(page->mapping->host, &key) >= 0) {
> +		cleancache_get_key(page->mapping->host, &key) >= 0) {
>  		(*cleancache_ops.put_page)(pool_id, key, page->index, page);
>  		cleancache_puts++;
>  	}
> @@ -153,19 +305,31 @@ EXPORT_SYMBOL(__cleancache_put_page);
>  /*
>   * Invalidate any data from cleancache associated with the poolid and the
>   * page's inode and page index so that a subsequent "get" will fail.
> + *
> + * The function has two checks before any action is taken - whether
> + * a backend is registered and whether the sb->cleancache_poolid
> + * is correct.
>   */
>  void __cleancache_invalidate_page(struct address_space *mapping,
>  					struct page *page)
>  {
>  	/* careful... page->mapping is NULL sometimes when this is called */
> -	int pool_id = mapping->host->i_sb->cleancache_poolid;
> +	int pool_id;
> +	int fake_pool_id = mapping->host->i_sb->cleancache_poolid;
>  	struct cleancache_filekey key = { .u.key = { 0 } };
>  
> -	if (pool_id >= 0) {
> +	if (!backend_registered)
> +		return;
> +
> +	if (fake_pool_id >= 0) {
> +		pool_id = get_poolid_from_fake(fake_pool_id);
> +		if (pool_id < 0)
> +			return;
> +
>  		VM_BUG_ON(!PageLocked(page));
>  		if (cleancache_get_key(mapping->host, &key) >= 0) {
>  			(*cleancache_ops.invalidate_page)(pool_id,
> -							  key, page->index);
> +					key, page->index);
>  			cleancache_invalidates++;
>  		}
>  	}
> @@ -176,12 +340,25 @@ EXPORT_SYMBOL(__cleancache_invalidate_page);
>   * Invalidate all data from cleancache associated with the poolid and the
>   * mappings's inode so that all subsequent gets to this poolid/inode
>   * will fail.
> + *
> + * The function has two checks before any action is taken - whether
> + * a backend is registered and whether the sb->cleancache_poolid
> + * is correct.
>   */
>  void __cleancache_invalidate_inode(struct address_space *mapping)
>  {
> -	int pool_id = mapping->host->i_sb->cleancache_poolid;
> +	int pool_id;
> +	int fake_pool_id = mapping->host->i_sb->cleancache_poolid;
>  	struct cleancache_filekey key = { .u.key = { 0 } };
>  
> +	if (!backend_registered)
> +		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);
>  }
> @@ -189,21 +366,37 @@ EXPORT_SYMBOL(__cleancache_invalidate_inode);
>  
>  /*
>   * Called by any cleancache-enabled filesystem at time of unmount;
> - * note that pool_id is surrendered and may be reutrned by a subsequent
> - * cleancache_init_fs or cleancache_init_shared_fs
> + * note that pool_id is surrendered and may be returned by a subsequent
> + * cleancache_init_fs or cleancache_init_shared_fs.
>   */
>  void __cleancache_invalidate_fs(struct super_block *sb)
>  {
> -	if (sb->cleancache_poolid >= 0) {
> -		int old_poolid = sb->cleancache_poolid;
> -		sb->cleancache_poolid = -1;
> -		(*cleancache_ops.invalidate_fs)(old_poolid);
> +	int index;
> +	int fake_pool_id = sb->cleancache_poolid;
> +	int old_poolid = fake_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 (backend_registered)
> +		(*cleancache_ops.invalidate_fs)(old_poolid);
> +	mutex_unlock(&poolid_mutex);
>  }
>  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)
> @@ -215,6 +408,11 @@ 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;
> +	}
> +	cleancache_enabled = 1;
>  	return 0;
>  }
>  module_init(init_cleancache)


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