linux-kernel - Re: [PATCH v3 5/7] libnvdimm: reduce an unnecessary if branch in nd_region

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Message-ID: <8027bff4-b100-2330-1bb6-5d0aa0215db3@huawei.com>
Date:   Thu, 27 Aug 2020 15:04:17 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Oliver O'Halloran <oohall@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Dave Jiang" <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Markus Elfring <Markus.Elfring@....de>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 5/7] libnvdimm: reduce an unnecessary if branch in
 nd_region_activate()

I will drop this patch, because I have a doubt:
Suppose the nd_region->ndr_mappings is 4, and for each nd_region->mapping[],
the value of num_flush is "0, 0, 4, 0", so the flush_data_size is "1 + 1 + 5 + 1", * sizeof(void *).
But in ndrd_get_flush_wpq() or ndrd_set_flush_wpq(), the expression is
"ndrd->flush_wpq[dimm * num + (hint & mask)]", I don't think the memory "ndrd" allocated is enough.
Please refer call chain: nd_region_activate() --> nvdimm_map_flush() --> ndrd_set_flush_wpq()

	for (i = 0; i < nd_region->ndr_mappings; i++) {
                struct nd_mapping *nd_mapping = &nd_region->mapping[i];
                struct nvdimm *nvdimm = nd_mapping->nvdimm;

                /* at least one null hint slot per-dimm for the "no-hint" case */
                flush_data_size += sizeof(void *);
                num_flush = min_not_zero(num_flush, nvdimm->num_flush);
                if (!nvdimm->num_flush)
                        continue;
                flush_data_size += nvdimm->num_flush * sizeof(void *);
        }

        ndrd = devm_kzalloc(dev, sizeof(*ndrd) + flush_data_size, GFP_KERNEL);




On 2020/8/20 10:16, Zhen Lei wrote:
> According to the original code logic:
> if (!nvdimm->num_flush) {
> 	flush_data_size += sizeof(void *);
> 	//nvdimm->num_flush is zero now, add 1) have no side effects
> } else {
> 	flush_data_size += sizeof(void *);
> 1)	flush_data_size += nvdimm->num_flush * sizeof(void *);
> }
> 
> Obviously, the above code snippet can be reduced to one statement:
> flush_data_size += (nvdimm->num_flush + 1) * sizeof(void *);
> 
> No functional change.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  drivers/nvdimm/region_devs.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index 7cf9c7d857909ce..49be115c9189eff 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -77,11 +77,8 @@ int nd_region_activate(struct nd_region *nd_region)
>  		}
>  
>  		/* at least one null hint slot per-dimm for the "no-hint" case */
> -		flush_data_size += sizeof(void *);
> +		flush_data_size += (nvdimm->num_flush + 1) * sizeof(void *);
>  		num_flush = min_not_zero(num_flush, nvdimm->num_flush);
> -		if (!nvdimm->num_flush)
> -			continue;
> -		flush_data_size += nvdimm->num_flush * sizeof(void *);
>  	}
>  	nvdimm_bus_unlock(&nd_region->dev);
>  
>