Re: [PATCH v6 01/45] mm: shrinker: add infrastructure for dynamically allocating shrinker

Muchun Song
2023-09-18T09:04:30Z

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@bytedance.com>
> ---
> 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;