Re: [External] Re: [PATCH v4 2/2] memory tier: create CPUless memory tiers after obtaining HMAT info

["Huang, Ying" <ying.huang@intel.com>]

"Ho-Ren (Jack) Chuang" <horenchuang@bytedance.com> writes:

> On Fri, Mar 22, 2024 at 1:41 AM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> "Ho-Ren (Jack) Chuang" <horenchuang@bytedance.com> writes:
>>
>> > The current implementation treats emulated memory devices, such as
>> > CXL1.1 type3 memory, as normal DRAM when they are emulated as normal memory
>> > (E820_TYPE_RAM). However, these emulated devices have different
>> > characteristics than traditional DRAM, making it important to
>> > distinguish them. Thus, we modify the tiered memory initialization process
>> > to introduce a delay specifically for CPUless NUMA nodes. This delay
>> > ensures that the memory tier initialization for these nodes is deferred
>> > until HMAT information is obtained during the boot process. Finally,
>> > demotion tables are recalculated at the end.
>> >
>> > * late_initcall(memory_tier_late_init);
>> > Some device drivers may have initialized memory tiers between
>> > `memory_tier_init()` and `memory_tier_late_init()`, potentially bringing
>> > online memory nodes and configuring memory tiers. They should be excluded
>> > in the late init.
>> >
>> > * Handle cases where there is no HMAT when creating memory tiers
>> > There is a scenario where a CPUless node does not provide HMAT information.
>> > If no HMAT is specified, it falls back to using the default DRAM tier.
>> >
>> > * Introduce another new lock `default_dram_perf_lock` for adist calculation
>> > In the current implementation, iterating through CPUlist nodes requires
>> > holding the `memory_tier_lock`. However, `mt_calc_adistance()` will end up
>> > trying to acquire the same lock, leading to a potential deadlock.
>> > Therefore, we propose introducing a standalone `default_dram_perf_lock` to
>> > protect `default_dram_perf_*`. This approach not only avoids deadlock
>> > but also prevents holding a large lock simultaneously.
>> >
>> > * Upgrade `set_node_memory_tier` to support additional cases, including
>> >   default DRAM, late CPUless, and hot-plugged initializations.
>> > To cover hot-plugged memory nodes, `mt_calc_adistance()` and
>> > `mt_find_alloc_memory_type()` are moved into `set_node_memory_tier()` to
>> > handle cases where memtype is not initialized and where HMAT information is
>> > available.
>> >
>> > * Introduce `default_memory_types` for those memory types that are not
>> >   initialized by device drivers.
>> > Because late initialized memory and default DRAM memory need to be managed,
>> > a default memory type is created for storing all memory types that are
>> > not initialized by device drivers and as a fallback.
>> >
>> > Signed-off-by: Ho-Ren (Jack) Chuang <horenchuang@bytedance.com>
>> > Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
>> > ---
>> >  mm/memory-tiers.c | 73 ++++++++++++++++++++++++++++++++++++++++-------
>> >  1 file changed, 63 insertions(+), 10 deletions(-)
>> >
>> > diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>> > index 974af10cfdd8..9396330fa162 100644
>> > --- a/mm/memory-tiers.c
>> > +++ b/mm/memory-tiers.c
>> > @@ -36,6 +36,11 @@ struct node_memory_type_map {
>> >
>> >  static DEFINE_MUTEX(memory_tier_lock);
>> >  static LIST_HEAD(memory_tiers);
>> > +/*
>> > + * The list is used to store all memory types that are not created
>> > + * by a device driver.
>> > + */
>> > +static LIST_HEAD(default_memory_types);
>> >  static struct node_memory_type_map node_memory_types[MAX_NUMNODES];
>> >  struct memory_dev_type *default_dram_type;
>> >
>> > @@ -108,6 +113,7 @@ static struct demotion_nodes *node_demotion __read_mostly;
>> >
>> >  static BLOCKING_NOTIFIER_HEAD(mt_adistance_algorithms);
>> >
>> > +static DEFINE_MUTEX(default_dram_perf_lock);
>>
>> Better to add comments about what is protected by this lock.
>>
>
> Thank you. I will add a comment like this:
> + /* The lock is used to protect `default_dram_perf*` info and nid. */
> +static DEFINE_MUTEX(default_dram_perf_lock);
>
> I also found an error path was not handled and
> found the lock could be put closer to what it protects.
> I will have them fixed in V5.
>
>> >  static bool default_dram_perf_error;
>> >  static struct access_coordinate default_dram_perf;
>> >  static int default_dram_perf_ref_nid = NUMA_NO_NODE;
>> > @@ -505,7 +511,8 @@ static inline void __init_node_memory_type(int node, struct memory_dev_type *mem
>> >  static struct memory_tier *set_node_memory_tier(int node)
>> >  {
>> >       struct memory_tier *memtier;
>> > -     struct memory_dev_type *memtype;
>> > +     struct memory_dev_type *mtype;
>>
>> mtype may be referenced without initialization now below.
>>
>
> Good catch! Thank you.
>
> Please check below.
> I may found a potential NULL pointer dereference.
>
>> > +     int adist = MEMTIER_ADISTANCE_DRAM;
>> >       pg_data_t *pgdat = NODE_DATA(node);
>> >
>> >
>> > @@ -514,11 +521,20 @@ static struct memory_tier *set_node_memory_tier(int node)
>> >       if (!node_state(node, N_MEMORY))
>> >               return ERR_PTR(-EINVAL);
>> >
>> > -     __init_node_memory_type(node, default_dram_type);
>> > +     mt_calc_adistance(node, &adist);
>> > +     if (node_memory_types[node].memtype == NULL) {
>> > +             mtype = mt_find_alloc_memory_type(adist, &default_memory_types);
>> > +             if (IS_ERR(mtype)) {
>> > +                     mtype = default_dram_type;
>> > +                     pr_info("Failed to allocate a memory type. Fall back.\n");
>> > +             }
>> > +     }
>> >
>> > -     memtype = node_memory_types[node].memtype;
>> > -     node_set(node, memtype->nodes);
>> > -     memtier = find_create_memory_tier(memtype);
>> > +     __init_node_memory_type(node, mtype);
>> > +
>> > +     mtype = node_memory_types[node].memtype;
>> > +     node_set(node, mtype->nodes);
>
> If the `mtype` could be NULL, would there be a potential
> NULL pointer dereference. Do you have a preferred idea
> to fix this if needed?

Initialize mtype with default_dram_type?

>> > +     memtier = find_create_memory_tier(mtype);
>> >       if (!IS_ERR(memtier))
>> >               rcu_assign_pointer(pgdat->memtier, memtier);
>> >       return memtier;
>> > @@ -655,6 +671,34 @@ void mt_put_memory_types(struct list_head *memory_types)
>> >  }
>> >  EXPORT_SYMBOL_GPL(mt_put_memory_types);
>> >

[snip]

--
Best Regards,
Huang, Ying