Re: [PATCH v6 6/8] powerpc/pmem: Avoid the barrier in flush routines

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 7/1/20 1:15 AM, Dan Williams wrote:
> On Tue, Jun 30, 2020 at 2:21 AM Aneesh Kumar K.V
> <aneesh.kumar@linux.ibm.com> wrote:
> [..]
>>>> The bio argument isn't for range based flushing, it is for flush
>>>> operations that need to complete asynchronously.
>>> How does the block layer determine that the pmem device needs
>>> asynchronous fushing?
>>>
>>
>>          set_bit(ND_REGION_ASYNC, &ndr_desc.flags);
>>
>> and dax_synchronous(dev)
> 
> Yes, but I think it is overkill to have an indirect function call just
> for a single instruction.
> 
> How about something like this instead, to share a common pmem_wmb()
> across x86 and powerpc.
> 
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index 20ff30c2ab93..b14009060c83 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -1180,6 +1180,13 @@ int nvdimm_flush(struct nd_region *nd_region,
> struct bio *bio)
>   {
>          int rc = 0;
> 
> +       /*
> +        * pmem_wmb() is needed to 'sfence' all previous writes such
> +        * that they are architecturally visible for the platform buffer
> +        * flush.
> +        */
> +       pmem_wmb();
> +
>          if (!nd_region->flush)
>                  rc = generic_nvdimm_flush(nd_region);
>          else {
> @@ -1206,17 +1213,14 @@ int generic_nvdimm_flush(struct nd_region *nd_region)
>          idx = this_cpu_add_return(flush_idx, hash_32(current->pid + idx, 8));
> 
>          /*
> -        * The first wmb() is needed to 'sfence' all previous writes
> -        * such that they are architecturally visible for the platform
> -        * buffer flush.  Note that we've already arranged for pmem
> -        * writes to avoid the cache via memcpy_flushcache().  The final
> -        * wmb() ensures ordering for the NVDIMM flush write.
> +        * Note that we've already arranged for pmem writes to avoid the
> +        * cache via memcpy_flushcache().  The final wmb() ensures
> +        * ordering for the NVDIMM flush write.
>           */
> -       wmb();


The series already convert this to pmem_wmb().

>          for (i = 0; i < nd_region->ndr_mappings; i++)
>                  if (ndrd_get_flush_wpq(ndrd, i, 0))
>                          writeq(1, ndrd_get_flush_wpq(ndrd, i, idx));
> -       wmb();
> +       pmem_wmb();


Should this be pmem_wmb()? This is ordering the above writeq() right?

> 
>          return 0;
>   }
> 

This still results in two pmem_wmb() on platforms that doesn't have 
flush_wpq. I was trying to avoid that by adding a nd_region->flush call 
back.

-aneesh
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