On Fri, Nov 16, 2012 at 03:10:49PM -0800, Andrew Morton wrote:
> On Wed, 14 Nov 2012 13:57:05 -0500
> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
>
> > From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> >
> > 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,
> > 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.
>
> What is your merge plan/path for this work?
>
> >
> > ...
> >
> > + * When no backend is registered all calls to init_fs and init_shard_fs
>
> "init_shared_fs"
>
> > + * are registered and fake poolids are given to the respective
> > + * super block but 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 fail.
>
> The comment could use all 80 cols..
>
> >
> > ...
> >
> > struct cleancache_ops cleancache_register_ops(struct cleancache_ops *ops)
> > {
> > struct cleancache_ops old = cleancache_ops;
> > + int i;
> >
> > 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);
> > + }
> > return old;
> > }
>
> I never noticed before - this function returns a large structure by
> value. That's really really unusual in the kernel. I see no problem
> with it per-se, but it might generate awful code.
>
> Also, this function has no locking and is blatantly racy.
>
> > EXPORT_SYMBOL(cleancache_register_ops);
> > @@ -61,15 +91,38 @@ 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;
> > +
> > + 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;
> > + }
> > + }
> > }
> > EXPORT_SYMBOL(__cleancache_init_fs);
>
> This also looks wildly racy.
>
> > /* 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;
> > +
> > + 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;
> > + }
> > + }
> > }
> > EXPORT_SYMBOL(__cleancache_init_shared_fs);
>
> Again, a huge mess if two threads execute this concurrently.
>
> > @@ -99,6 +152,19 @@ static int cleancache_get_key(struct inode *inode,
> > }
> >
> > /*
> > + * Returns a pool_id that is associated with a given fake poolid.
>
> Was there a comment anywhere which tells the reader what a "fake poolid" is?
>
> > + */
> > +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.
> > @@ -109,17 +175,26 @@ 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;
> > + }
>
> Races everywhere...
>
> > 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
> > @@ -138,12 +213,23 @@ 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 (!backend_registered) {
> > + cleancache_puts++;
> > + return;
> > + }
>
> More..
>
> > 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++;
> > }
> >
> > ...
> >
>
> I don't understand the timing flow here, nor the existing constraints
> on what can be done and when, but....
>
> The whole thing looks really hacky? Why do we need to remember all
> this stuff for later on? What prevents us from simply synchonously
> doing whatever we need to do when someone wants to register a backend?
>
> Maybe a little ascii time chart/flow diagram would help.
This patch hopefully answers the questions and comments. It has a bit
of a a), then b), then c) type chart to illustrate the issue of backends
registered asynchronously.
>From aa306144b1be66cc3e53299a454e8fe2506e19c0 Mon Sep 17 00:00:00 2001
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 14 Nov 2012 13:57:05 -0500
Subject: [PATCH 1/8] mm: cleancache: lazy initialization to allow tmem
backends to build/run as modules
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,
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@xxxxxxxxxxxxxx>
Signed-off-by: Florian Schmaus <fschmaus@xxxxxxxxx>
Signed-off-by: Andor Daam <andor.daam@xxxxxxxxxxxxxx>
Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
[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@xxxxxxxxxx>
---
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
+ * 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)
--
1.7.11.7
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