Re: [PATCH v6 01/45] mm: shrinker: add infrastructure for dynamically allocating shrinker
From: Muchun Song
Date: Mon Sep 18 2023 - 05:04:39 EST
On 2023/9/11 17:44, Qi Zheng wrote:
Currently, the shrinker instances can be divided into the following three
types:
a) global shrinker instance statically defined in the kernel, such as
workingset_shadow_shrinker.
b) global shrinker instance statically defined in the kernel modules, such
as mmu_shrinker in x86.
c) shrinker instance embedded in other structures.
For case a, the memory of shrinker instance is never freed. For case b,
the memory of shrinker instance will be freed after synchronize_rcu() when
the module is unloaded. For case c, the memory of shrinker instance will
be freed along with the structure it is embedded in.
In preparation for implementing lockless slab shrink, we need to
dynamically allocate those shrinker instances in case c, then the memory
can be dynamically freed alone by calling kfree_rcu().
So this commit adds the following new APIs for dynamically allocating
shrinker, and add a private_data field to struct shrinker to record and
get the original embedded structure.
1. shrinker_alloc()
Used to allocate shrinker instance itself and related memory, it will
return a pointer to the shrinker instance on success and NULL on failure.
2. shrinker_register()
Used to register the shrinker instance, which is same as the current
register_shrinker_prepared().
3. shrinker_free()
Used to unregister (if needed) and free the shrinker instance.
In order to simplify shrinker-related APIs and make shrinker more
independent of other kernel mechanisms, subsequent submissions will use
the above API to convert all shrinkers (including case a and b) to
dynamically allocated, and then remove all existing APIs.
This will also have another advantage mentioned by Dave Chinner:
```
The other advantage of this is that it will break all the existing
out of tree code and third party modules using the old API and will
no longer work with a kernel using lockless slab shrinkers. They
need to break (both at the source and binary levels) to stop bad
things from happening due to using unconverted shrinkers in the new
setup.
```
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
include/linux/shrinker.h | 7 +++
mm/internal.h | 11 +++++
mm/shrinker.c | 102 +++++++++++++++++++++++++++++++++++++++
mm/shrinker_debug.c | 17 ++++++-
4 files changed, 135 insertions(+), 2 deletions(-)
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 6b5843c3b827..3f3fd9974ce5 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -70,6 +70,8 @@ struct shrinker {
int seeks; /* seeks to recreate an obj */
unsigned flags;
+ void *private_data;
+
/* These are for internal use */
struct list_head list;
#ifdef CONFIG_MEMCG
@@ -95,6 +97,11 @@ struct shrinker {
* non-MEMCG_AWARE shrinker should not have this flag set.
*/
#define SHRINKER_NONSLAB (1 << 3)
+#define SHRINKER_ALLOCATED (1 << 4)
It is better to add a comment here to tell users
it is only used by internals of shrinker. The users
are not supposed to pass this flag to shrink APIs.
+
+struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
+void shrinker_register(struct shrinker *shrinker);
+void shrinker_free(struct shrinker *shrinker);
extern int __printf(2, 3) prealloc_shrinker(struct shrinker *shrinker,
const char *fmt, ...);
diff --git a/mm/internal.h b/mm/internal.h
index 0471d6326d01..5587cae20ebf 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1161,6 +1161,9 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
#ifdef CONFIG_SHRINKER_DEBUG
extern int shrinker_debugfs_add(struct shrinker *shrinker);
+extern int shrinker_debugfs_name_alloc(struct shrinker *shrinker,
+ const char *fmt, va_list ap);
+extern void shrinker_debugfs_name_free(struct shrinker *shrinker);
extern struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
int *debugfs_id);
extern void shrinker_debugfs_remove(struct dentry *debugfs_entry,
@@ -1170,6 +1173,14 @@ static inline int shrinker_debugfs_add(struct shrinker *shrinker)
{
return 0;
}
+static inline int shrinker_debugfs_name_alloc(struct shrinker *shrinker,
+ const char *fmt, va_list ap)
+{
+ return 0;
+}
+static inline void shrinker_debugfs_name_free(struct shrinker *shrinker)
+{
+}
static inline struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
int *debugfs_id)
{
diff --git a/mm/shrinker.c b/mm/shrinker.c
index a16cd448b924..201211a67827 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -550,6 +550,108 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
return freed;
}
+struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...)
+{
+ struct shrinker *shrinker;
+ unsigned int size;
+ va_list ap;
+ int err;
+
+ shrinker = kzalloc(sizeof(struct shrinker), GFP_KERNEL);
+ if (!shrinker)
+ return NULL;
+
+ va_start(ap, fmt);
+ err = shrinker_debugfs_name_alloc(shrinker, fmt, ap);
+ va_end(ap);
+ if (err)
+ goto err_name;
+
+ shrinker->flags = flags | SHRINKER_ALLOCATED;
+ shrinker->seeks = DEFAULT_SEEKS;
+
+ if (flags & SHRINKER_MEMCG_AWARE) {
+ err = prealloc_memcg_shrinker(shrinker);
+ if (err == -ENOSYS)
+ shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
+ else if (err == 0)
+ goto done;
+ else
+ goto err_flags;
Actually, the code here is a little confusing me when I fist look
at it. I think there could be some improvements here. Something
like:
if (flags & SHRINKER_MEMCG_AWARE) {
err = prealloc_memcg_shrinker(shrinker);
if (err == -ENOSYS) {
/* Memcg is not supported and fallback to
non-memcg-aware shrinker. */
shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
goto non-memcg;
}
if (err)
goto err_flags;
return shrinker;
}
non-memcg:
[...]
return shrinker;
In this case, the code becomes more clear (at least for me). We have
split the
code into two part, one is handling memcg-aware case, another is
non-memcg-aware
case. Any side will have a explicit "return" keyword to return once
succeeds.
It is a little implicit that the previous one uses "goto done".
And the tag of "non-memcg" is also a good annotation to tell us the
following
code handles non-memcg-aware case.
+ }
+
+ /*
+ * The nr_deferred is available on per memcg level for memcg aware
+ * shrinkers, so only allocate nr_deferred in the following cases:
+ * - non memcg aware shrinkers
+ * - !CONFIG_MEMCG
+ * - memcg is disabled by kernel command line
+ */
+ size = sizeof(*shrinker->nr_deferred);
+ if (flags & SHRINKER_NUMA_AWARE)
+ size *= nr_node_ids;
+
+ shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
+ if (!shrinker->nr_deferred)
+ goto err_flags;
+
+done:
+ return shrinker;
+
+err_flags:
+ shrinker_debugfs_name_free(shrinker);
+err_name:
+ kfree(shrinker);
+ return NULL;
+}
+EXPORT_SYMBOL_GPL(shrinker_alloc);
+
+void shrinker_register(struct shrinker *shrinker)
+{
+ if (unlikely(!(shrinker->flags & SHRINKER_ALLOCATED))) {
+ pr_warn("Must use shrinker_alloc() to dynamically allocate the shrinker");
+ return;
+ }
+
+ down_write(&shrinker_rwsem);
+ list_add_tail(&shrinker->list, &shrinker_list);
+ shrinker->flags |= SHRINKER_REGISTERED;
+ shrinker_debugfs_add(shrinker);
+ up_write(&shrinker_rwsem);
+}
+EXPORT_SYMBOL_GPL(shrinker_register);
+
+void shrinker_free(struct shrinker *shrinker)
+{
+ struct dentry *debugfs_entry = NULL;
+ int debugfs_id;
+
+ if (!shrinker)
+ return;
+
+ down_write(&shrinker_rwsem);
+ if (shrinker->flags & SHRINKER_REGISTERED) {
+ list_del(&shrinker->list);
+ debugfs_entry = shrinker_debugfs_detach(shrinker, &debugfs_id);
+ shrinker->flags &= ~SHRINKER_REGISTERED;
+ } else {
+ shrinker_debugfs_name_free(shrinker);
We could remove shrinker_debugfs_name_free() calling from
shrinker_debugfs_detach(), then we could call
shrinker_debugfs_name_free() anyway, otherwise, it it a little
weird for me. And the srinker name is allocated from shrinker_alloc(),
so I think it it reasonable for shrinker_free() to free the
shrinker name.
Thanks.
+ }
+
+ if (shrinker->flags & SHRINKER_MEMCG_AWARE)
+ unregister_memcg_shrinker(shrinker);
+ up_write(&shrinker_rwsem);
+
+ if (debugfs_entry)
+ shrinker_debugfs_remove(debugfs_entry, debugfs_id);
+
+ kfree(shrinker->nr_deferred);
+ shrinker->nr_deferred = NULL;
+
+ kfree(shrinker);
+}
+EXPORT_SYMBOL_GPL(shrinker_free);
+
/*
* Add a shrinker callback to be called from the vm.
*/
diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index e4ce509f619e..38452f539f40 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -193,6 +193,20 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
return 0;
}
+int shrinker_debugfs_name_alloc(struct shrinker *shrinker, const char *fmt,
+ va_list ap)
+{
+ shrinker->name = kvasprintf_const(GFP_KERNEL, fmt, ap);
+
+ return shrinker->name ? 0 : -ENOMEM;
+}
+
+void shrinker_debugfs_name_free(struct shrinker *shrinker)
+{
+ kfree_const(shrinker->name);
+ shrinker->name = NULL;
+}
It it better to move both helpers to internal.h and mark them as inline
since both are very simple enough.
Thanks.
+
int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
{
struct dentry *entry;
@@ -241,8 +255,7 @@ struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
lockdep_assert_held(&shrinker_rwsem);
- kfree_const(shrinker->name);
- shrinker->name = NULL;
+ shrinker_debugfs_name_free(shrinker);
*debugfs_id = entry ? shrinker->debugfs_id : -1;
shrinker->debugfs_entry = NULL;