On Mon, 29 Aug 2022 22:30:49 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
> At present, both THP and KSM module have similar structures
> mm_slot for organizing and recording the information required
> for scanning mm, and each defines the following exactly the
> same operation functions:
>
> - alloc_mm_slot
> - free_mm_slot
> - get_mm_slot
> - insert_to_mm_slots_hash
>
> In order to de-duplicate these codes, this patch introduces a
> common struct mm_slot, and subsequent patches will let THP and
> KSM to use it.
Seems like a good idea.
> --- /dev/null
> +++ b/mm/mm_slot.h
> @@ -0,0 +1,55 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#ifndef _LINUX_MM_SLOT_H
> +#define _LINUX_MM_SLOT_H
> +
> +#include <linux/hashtable.h>
> +#include <linux/slab.h>
> +
> +/*
> + * struct mm_slot - hash lookup from mm to mm_slot
> + * @hash: link to the mm_slots hash list
> + * @mm_node: link into the mm_slots list
> + * @mm: the mm that this information is valid for
> + */
> +struct mm_slot {
> + struct hlist_node hash;
> + struct list_head mm_node;
> + struct mm_struct *mm;
> +};
It appears that the presence of an mm_struct in the hash list does not
contribute to the mm_struct's refcount? That's somewhat unexpected.
It would be helpful to add some words here describing the means by
which a user of mm_slot would prevent the mm_struct from getting freed
while on the list. I assume "caller must maintain a reference on the
mm_struct while it remains on an mm_slot hash list"?
> +#define mm_slot_entry(ptr, type, member) \
> + container_of(ptr, type, member)
> +
> +static inline void *alloc_mm_slot(struct kmem_cache *cache)
> +{
> + if (!cache) /* initialization failed */
> + return NULL;
> + return kmem_cache_zalloc(cache, GFP_KERNEL);
> +}
> +
> +static inline void free_mm_slot(struct kmem_cache *cache, void *objp)
> +{
> + kmem_cache_free(cache, objp);
> +}
> +
> +#define get_mm_slot(_hashtable, _mm) \
> +({ \
> + struct mm_slot *tmp_slot, *mm_slot = NULL; \
> + \
> + hash_for_each_possible(_hashtable, tmp_slot, hash, (unsigned long)_mm) \
> + if (_mm == tmp_slot->mm) { \
> + mm_slot = tmp_slot; \
> + break; \
> + } \
> + \
> + mm_slot; \
> +})
Is there a reason why this must be implemented as a macro? That's
preferable, although this may be overly large for inlining. mm/util.c
might suit.
> +#define insert_to_mm_slots_hash(_hashtable, _mm, _mm_slot) \
> +({ \
> + _mm_slot->mm = _mm; \
> + hash_add(_hashtable, &_mm_slot->hash, (unsigned long)_mm); \
> +})
Does this need to be a macro?
And the naming. Can we please have
mm_slot_entry
mm_slot_alloc
mm_slot_free
mm_slot_get
mm_slot_insert
Also, "get" usually implies that a refcout is taken on the obtained
object, so mm_slot_lookup() would be more appropriate.
