Re: [PATCH v10 1/2] memory tier: dax/kmem: introduce an abstract layer for finding, allocating, and putting memory types

Jonathan Cameron
2024-04-03T16:52:06Z

On Tue, 2 Apr 2024 00:17:37 +0000
"Ho-Ren (Jack) Chuang" <horenchuang@bytedance.com> wrote:

> Since different memory devices require finding, allocating, and putting
> memory types, these common steps are abstracted in this patch,
> enhancing the scalability and conciseness of the code.
>
> Signed-off-by: Ho-Ren (Jack) Chuang <horenchuang@bytedance.com>
> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Hi,

I know this is a late entry to the discussion but a few comments inline.
(sorry I didn't look earlier!)

All opportunities to improve code complexity and readability as a result
of your factoring out.

Jonathan

> ---
> drivers/dax/kmem.c | 20 ++------------------
> include/linux/memory-tiers.h | 13 +++++++++++++
> mm/memory-tiers.c | 32 ++++++++++++++++++++++++++++++++
> 3 files changed, 47 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
> index 42ee360cf4e3..01399e5b53b2 100644
> --- a/drivers/dax/kmem.c
> +++ b/drivers/dax/kmem.c
> @@ -55,21 +55,10 @@ static LIST_HEAD(kmem_memory_types);
>
> static struct memory_dev_type *kmem_find_alloc_memory_type(int adist)
> {
> - bool found = false;
> struct memory_dev_type *mtype;
>
> mutex_lock(&kmem_memory_type_lock);
could use

guard(mutex)(&kmem_memory_type_lock);
return mt_find_alloc_memory_type(adist, &kmem_memory_types);

I'm fine if you ignore this comment though as may be other functions in
here that could take advantage of the cleanup.h stuff in a future patch.

> - list_for_each_entry(mtype, &kmem_memory_types, list) {
> - if (mtype->adistance == adist) {
> - found = true;
> - break;
> - }
> - }
> - if (!found) {
> - mtype = alloc_memory_type(adist);
> - if (!IS_ERR(mtype))
> - list_add(&mtype->list, &kmem_memory_types);
> - }
> + mtype = mt_find_alloc_memory_type(adist, &kmem_memory_types);
> mutex_unlock(&kmem_memory_type_lock);
>
> return mtype;

> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
> index 69e781900082..a44c03c2ba3a 100644
> --- a/include/linux/memory-tiers.h
> +++ b/include/linux/memory-tiers.h
> @@ -48,6 +48,9 @@ int mt_calc_adistance(int node, int *adist);
> int mt_set_default_dram_perf(int nid, struct access_coordinate *perf,
> const char *source);
> int mt_perf_to_adistance(struct access_coordinate *perf, int *adist);
> +struct memory_dev_type *mt_find_alloc_memory_type(int adist,
> + struct list_head *memory_types);

That indent looks unusual. Align the start of struct with start of int.

> +void mt_put_memory_types(struct list_head *memory_types);
> #ifdef CONFIG_MIGRATION
> int next_demotion_node(int node);
> void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets);
> @@ -136,5 +139,15 @@ static inline int mt_perf_to_adistance(struct access_coordinate *perf, int *adis
> {
> return -EIO;
> }
> +
> +struct memory_dev_type *mt_find_alloc_memory_type(int adist, struct list_head *memory_types)
> +{
> + return NULL;
> +}
> +
> +void mt_put_memory_types(struct list_head *memory_types)
> +{
> +
No blank line needed here.
> +}
> #endif /* CONFIG_NUMA */
> #endif /* _LINUX_MEMORY_TIERS_H */
> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> index 0537664620e5..974af10cfdd8 100644
> --- a/mm/memory-tiers.c
> +++ b/mm/memory-tiers.c
> @@ -623,6 +623,38 @@ void clear_node_memory_type(int node, struct memory_dev_type *memtype)
> }
> EXPORT_SYMBOL_GPL(clear_node_memory_type);
>
> +struct memory_dev_type *mt_find_alloc_memory_type(int adist, struct list_head *memory_types)

Breaking this out as a separate function provides opportunity to improve it.
Maybe a follow up patch makes sense given it would no longer be a straight
forward code move. However in my view it would be simple enough to be obvious
even within this patch.

> +{
> + bool found = false;
> + struct memory_dev_type *mtype;
> +
> + list_for_each_entry(mtype, memory_types, list) {
> + if (mtype->adistance == adist) {
> + found = true;

Why not return here?
return mtype;

> + break;
> + }
> + }
> + if (!found) {

If returning above, no need for found variable - just do this unconditionally.
+ I suggest you flip logic for simpler to follow code flow.
It's more code but I think a bit easier to read as error handling is
out of the main simple flow.

mtype = alloc_memory_type(adist);
if (IS_ERR(mtype))
return mtype;

list_add(&mtype->list, memory_types);

return mtype;

> + mtype = alloc_memory_type(adist);
> + if (!IS_ERR(mtype))
> + list_add(&mtype->list, memory_types);
> + }
> +
> + return mtype;
> +}
> +EXPORT_SYMBOL_GPL(mt_find_alloc_memory_type);
> +
> +void mt_put_memory_types(struct list_head *memory_types)
> +{
> + struct memory_dev_type *mtype, *mtn;
> +
> + list_for_each_entry_safe(mtype, mtn, memory_types, list) {
> + list_del(&mtype->list);
> + put_memory_type(mtype);
> + }
> +}
> +EXPORT_SYMBOL_GPL(mt_put_memory_types);
> +
> static void dump_hmem_attrs(struct access_coordinate *coord, const char *prefix)
> {
> pr_info(