Re: [PATCH v3 5/7] libnvdimm: reduce an unnecessary if branch in nd_region_activate()

Leizhen (ThunderTown)
2020-08-27T07:04:33Z

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@huawei.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);
>
>