Re: [PATCH 1/7] mm: introduce common struct mm_slot

[Yang Shi <shy828301@gmail.com>]

On Mon, Aug 29, 2022 at 12:51 PM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Mon, 29 Aug 2022 22:30:49 +0800 Qi Zheng <zhengqi.arch@bytedance.com> 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.

I didn't find time to look into the series yet, but when the
mm/mm_slot was added to the list, mmgrab() was definitely called if
this was not changed by the series.

>
> 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.
>