Question

我正在研究内核模块，我需要比较两个缓冲区以确定它们是否相同。我正在使用Linux内核中定义的memcmp函数来执行此操作。我的第一个缓冲区是这样的：

cache_buffer = (unsigned char *)vmalloc(4097);
cache_buffer[4096] = '/0';

第二个缓冲区来自使用page_address（）函数的页面。

page = bio_page(bio);
kmap(page);
write_buffer = (char *)page_address(page);
kunmap(page);

我已经预先打印了两个缓冲区的内容，不仅打印正确，而且它们也有相同的内容。接下来，我这样做：

result = memcmp(write_buffer, cache_buffer, 2048); // only comparing up to 2048 positions

这导致内核冻结，我无法弄清楚原因。我检查了memcmp的实现，没有看到任何会导致冻结的东西。任何人都可以建议一个原因吗？

这是memcmp实现：

int memcmp(const void *cs, const void *ct, size_t count)
{
    const unsigned char *su1, *su2;
    int res = 0;

    for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
           if ((res = *su1 - *su2) != 0)
                     break;
    return res;
}

编辑：导致冻结的功能是memcmp。当我评论它时，一切都有效。另外，当我做了memcmp时，如下

memcmp(write_buffer, write_buffer, 2048); //comparing two write_buffers

一切都运作良好。只有当我将cache_buffer抛入混合时才会出现错误。此外，上面是我的实际代码的简化。这是整个功能：

static int compare_data(sector_t location, struct bio * bio, struct cache_c * dmc)
{
struct dm_io_region where;
unsigned long bits;
int segno;
struct bio_vec * bvec;
struct page * page;
unsigned char * cache_data;
    char * temp_data;
char * write_data;
int result, length, i;

cache_data = (unsigned char *)vmalloc((dmc->block_size * 512) + 1);

where.bdev = dmc->cache_dev->bdev;
where.count = dmc->block_size;
where.sector = location << dmc->block_shift;

printk(KERN_DEBUG "place: %llu\n", where.sector);

dm_io_sync_vm(1, &where, READ, cache_data, &bits, dmc);

length = 0;

bio_for_each_segment(bvec, bio, segno)
{
    if(segno == 0)
    {
        page = bio_page(bio);
        kmap(page);
        write_data = (char *)page_address(page);
        //kunmap(page);
        length += bvec->bv_len;
    }
    else
    {
        page = bio_page(bio);
        kmap(page);
        temp_data = strcat(write_data, (char *)page_address(page));
        //kunmap(page);
        write_data = temp_data;
        length += bvec->bv_len;
    }
}

printk(KERN_INFO "length: %u\n", length);

cache_data[dmc->block_size * 512] = '\0';

for(i = 0; i < 2048; i++)
{
    printk("%c", write_data[i]);
}

printk("\n");

for(i = 0; i < 2048; i++)
{
    printk("%c", cache_data[i]);
}

printk("\n");    

result = memcmp(write_data, cache_data, length);

return result;    
}

编辑＃2：对不起伙计们。问题不在于memcmp。这是memcmp的结果。当它返回正数或负数时，调用我的函数的函数将使用一些指针，其中一个未初始化。我不知道为什么我之前没有意识到这一点。感谢您尽力提供帮助！

Answer 1

我不是内核专家，但我认为你需要在进行比较时保留这个内存映射？换句话说，在kunmap完成之前，请勿致电memcmp。我认为之前调用它会导致write_buffer指向不再映射的页面。

在另一个问题中使用您的代码，这是对增量的粗略尝试。仍然需要一些清理，我敢肯定：

static int compare_data(sector_t location, struct bio * bio, struct cache_c * dmc)
{
  struct dm_io_region where;
  unsigned long bits;
  int segno;
  struct bio_vec * bvec;
  struct page * page;
  unsigned char * cache_data;
  char * temp_data;
  char * write_data;
  int length, i;

  int result = 0;
  size_t position = 0;
  size_t max_size = (dmc->block_size * 512) + 1;

  cache_data = (unsigned char *)vmalloc(max_size);

  where.bdev = dmc->cache_dev->bdev;
  where.count = dmc->block_size;
  where.sector = location << dmc->block_shift;

  printk(KERN_DEBUG "place: %llu\n", where.sector);

  dm_io_sync_vm(1, &where, READ, cache_data, &bits, dmc);

  bio_for_each_segment(bvec, bio, segno)
  {
    // Map the page into memory
    page = bio_page(bio);
    write_data = (char *)kmap(page);
    length = bvec->bv_len;

    // Make sure we don't go past the end
    if(position >= max_size)
      break;
    if(position + length > max_size)
      length = max_size - position;

    // Compare the data
    result = memcmp(write_data, cache_data + position, length);
    position += length;

    kunmap(page);

    // If the memory is not equal, bail out now and return the result
    if(result != 0)
      break;
  }

  cache_data[dmc->block_size * 512] = '\0';

  return result;    
}

内存比较导致系统停止

1 个答案: