linux-kernel - Re: [PATCH -V2] cxl/region: Support to calculate memory tier abstract distance

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Date: Mon, 17 Jun 2024 10:10:05 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Alison Schofield <alison.schofield@...el.com>,  Dave Jiang
 <dave.jiang@...el.com>
Cc: Dan Williams <dan.j.williams@...el.com>,  <linux-cxl@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>,  Andrew Morton
 <akpm@...ux-foundation.org>,  Jonathan Cameron
 <Jonathan.Cameron@...wei.com>,  Bharata B Rao <bharata@....com>,  Alistair
 Popple <apopple@...dia.com>,  "Aneesh Kumar K . V"
 <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH -V2] cxl/region: Support to calculate memory tier
 abstract distance

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

> On Tue, Jun 11, 2024 at 01:54:23PM +0800, Ying Huang wrote:

[snip]

>> ---
>>  drivers/cxl/core/region.c | 40 +++++++++++++++++++++++++++++++++++----
>>  drivers/cxl/cxl.h         |  2 ++
>>  2 files changed, 38 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 3c2b6144be23..81d0910c0a02 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -9,6 +9,7 @@
>>  #include <linux/uuid.h>
>>  #include <linux/sort.h>
>>  #include <linux/idr.h>
>> +#include <linux/memory-tiers.h>
>>  #include <cxlmem.h>
>>  #include <cxl.h>
>>  #include "core.h"
>> @@ -2304,14 +2305,20 @@ static bool cxl_region_update_coordinates(struct cxl_region *cxlr, int nid)
>>  	return true;
>>  }
>>  
>> +static int cxl_region_nid(struct cxl_region *cxlr)
>> +{
>> +	struct cxl_region_params *p = &cxlr->params;
>> +	struct cxl_endpoint_decoder *cxled = p->targets[0];
>> +	struct cxl_decoder *cxld = &cxled->cxld;
>> +
>> +	return phys_to_target_node(cxld->hpa_range.start);
>> +}
>> +
>
> I believe it's OK to send a resource_size_t to phys_to_target_node()
> like this:
>
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -2308,10 +2308,8 @@ static bool cxl_region_update_coordinates(struct cxl_region *cxlr, int nid)
>  static int cxl_region_nid(struct cxl_region *cxlr)
>  {
>         struct cxl_region_params *p = &cxlr->params;
> -       struct cxl_endpoint_decoder *cxled = p->targets[0];
> -       struct cxl_decoder *cxld = &cxled->cxld;
>
> -       return phys_to_target_node(cxld->hpa_range.start);
> +       return phys_to_target_node(p->res->start);
>  }
>

Read the related code again, it appears that there's a theoretical race
condition here.  The register_memory_notifier() is called in
devm_cxl_add_region(), where p->targets[] and p->res haven't been
setupped yet.  And, IIUC, p->targets[] or p->res may be gone during the
life cycle of regions too.  If so, we need to use
guard(rwsem_read)(&cxl_region_rwsem) to protect p->targets[] and p->res
references.  Because the memory notifier may be called for other nodes
online/offline.

--
Best Regards,
Huang, Ying