PMEM Block Driver in Kernel

[J Kim]

Hello, 

I'm Junghan, a graduate student in South Korea. 

I'm currently doing study on NVDIMM software stack in Linux Kernel. 

There is a block driver called PMEM (driver/nvdimm/pmem.c), it look likes Ramdisk. 

I am verifying DRAM to NVM copy operation and its performance. 

First, I want to check the amount of data difference using XOR operation during BIO operation. 

Strangely, there are a lot of block with the same SRC (mem) and DST(pmem_addr).  (below: code examples)

I think that simple 8 byte XOR operation code has a problem or this is because of DRAM emulation.. 

I still couldn't figure out why this is happening.  

Please give me some advice.

- Linux Kernel 4.13 

- NVDIMM : DRAM Emulation (link) 

Thanks ..

/driver/nvdimm/pmem.c


static blk_status_t pmem_do_bvec(struct pmem_device *pmem, struct page *page,
                unsigned int len, unsigned int off, bool is_write,
                sector_t sector)
{
   void *mem = kmap_atomic(page);
   phys_addr_t pmem_off = sector * 512 + pmem->data_offset;
   void *pmem_addr = pmem->virt_addr + pmem_off;


   long long int *src = mem; /*8 byte*/
   long long int *dst = pmem_addr;

    ....

   if(write) {
      for (i=0; i<(PAGE_SIZE / 8);i++){
           if ((src[i] ^ dst[i]) == 0){ /* 8 Byte XOR, 8 * 512 = 4096, PAGE_SIZE */
                diff_count--;
           }
      }
      if (diff_count == 0) {
            // SRC and DST are Same block
            pr_warn("[%d] SAME Block: Write Count, %u\n", idx, binfo->writecount);    
      }
   }
     ....
}