An effective DRAM address remapping for mitigating rowhammer errors

A rowhammer error represents a loss of data stored in a DRAM cell caused by electromagnetic interference due to repetitive access to the same and/or adjacent rows. Due to the concentrated occurrence of rowhammer errors in specific rows and columns, these errors cannot be corrected by the conventional error correcting code (ECC) commonly used in DRAM devices. Previous techniques avoid these errors by having additional refresh operations that require additional hardware resources and/or power consumption. This paper proposes a different approach to handle rowhammer errors by distributing them across different DRAM rows and columns so that the attack cells are not concentrated on specific rows and columns. To this end, the distribution of rowhammer errors is observed with experiments using several commercial DRAM devices by employing state-of-the-art rowhammer attack techniques. The observation of the rowhammer errors concentrated in specific rows and columns underlies the proposal of an effective DRAM address remapping scheme for re-distribution of rowhammer errors. By using different address mappings to different chips and arrays in a DIMM, the proposed remapping effectively distributes errors over different rows and columns. As a result, the proposed remapping scheme decreases the possibility of multiple errors in a single word, and consequently, reduces uncorrectable errors under single error or single symbol correcting ECC. Experimental results with commercial DIMMs show that the proposed scheme reduces uncorrectable errors by about 95 percent while incurring a small additional hardware cost.