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From:   Tarun Sahu <tsahu@linux.ibm.com>
To:     Alison Schofield <alison.schofield@intel.com>
Cc:     nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org,
        linux-kernel@vger.kernel.org, dave.jiang@intel.com,
        dan.j.williams@intel.com, vishal.l.verma@intel.com,
        aneesh.kumar@linux.ibm.com, jaypatel@linux.ibm.com
Subject: Re: [PATCH] dax/kmem: Pass valid argument to
 memory_group_register_static
In-Reply-To: <ZJIz07hQiXr/MghO@aschofie-mobl2>
References: <20230620140332.30578-1-tsahu@linux.ibm.com>
 <ZJIz07hQiXr/MghO@aschofie-mobl2>
Date:   Wed, 21 Jun 2023 11:36:59 +0530
Message-ID: <87cz1p1h8c.fsf@linux.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Hi Alison,

Alison Schofield <alison.schofield@intel.com> writes:

> On Tue, Jun 20, 2023 at 07:33:32PM +0530, Tarun Sahu wrote:
>> memory_group_register_static takes maximum number of pages as the argument
>> while dev_dax_kmem_probe passes total_len (in bytes) as the argument.
>
> This sounds like a fix. An explanation of the impact and a fixes tag
> may be needed. Also, wondering how you found it.
>
Yes, it is a fix, I found it during dry code walk-through.
There is not any impact as such. As,
memory_group_register_static just set the max_pages limit which
is used in auto_movable_zone_for_pfn to determine the zone.

which might cause these condition to behave differently,

This will be true always so jump will happen to kernel_zone
	if (!auto_movable_can_online_movable(NUMA_NO_NODE, group, nr_pages))
		goto kernel_zone;
---
kernel_zone:
	return default_kernel_zone_for_pfn(nid, pfn, nr_pages);

---

Here, In below, zone_intersects compare range will be larger as nr_pages
will be higher (derived from total_len passed in dev_dax_kmem_probe).

static struct zone *default_kernel_zone_for_pfn(int nid, unsigned long start_pfn,
		unsigned long nr_pages)
{
	struct pglist_data *pgdat = NODE_DATA(nid);
	int zid;

	for (zid = 0; zid < ZONE_NORMAL; zid++) {
		struct zone *zone = &pgdat->node_zones[zid];

		if (zone_intersects(zone, start_pfn, nr_pages))
			return zone;
	}

	return &pgdat->node_zones[ZONE_NORMAL];
}

In Mostly cases, ZONE_NORMAL will be returned. But there is no
crash/panic issues involved here, only decision making on selecting zone
is affected.


> Alison
>
>> 
>> Signed-off-by: Tarun Sahu <tsahu@linux.ibm.com>
>> ---
>>  drivers/dax/kmem.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
>> index 7b36db6f1cbd..898ca9505754 100644
>> --- a/drivers/dax/kmem.c
>> +++ b/drivers/dax/kmem.c
>> @@ -99,7 +99,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>  	if (!data->res_name)
>>  		goto err_res_name;
>>  
>> -	rc = memory_group_register_static(numa_node, total_len);
>> +	rc = memory_group_register_static(numa_node, PFN_UP(total_len));
>>  	if (rc < 0)
>>  		goto err_reg_mgid;
>>  	data->mgid = rc;
>> -- 
>> 2.31.1
>>