On Thu, Jan 28, 2016 at 10:12 AM, Toshi Kani <[email protected]> wrote: > Data corruption issues were observed in tests which initiated a system > crash/reset while accessing BTT devices. This problem is reproducible. > > The BTT driver calls pmem_rw_bytes() to update data in pmem devices. > This interface calls __copy_user_nocache(), which uses non-temporal > stores so that the stores to pmem are persistent. > > __copy_user_nocache() uses non-temporal stores when a request size is > 8 bytes or larger (and is aligned by 8 bytes). The BTT driver updates > the BTT map table, which entry size is 4 bytes. Therefore, updates to > the map table entries remain cached, and are not written to pmem after > a crash. Since the BTT driver makes previous blocks free and uses them > for subsequent writes, the map table ends up pointing to blocks allocated > for other LBAs after a crash. > > Patch 1 extends __copy_user_nocache() to use non-temporal store for > 4 byte copy. This patch fixes the BTT data corruption issue. >
Nice find! > Patch 2 changes arch_memcpy_to_pmem() to flush processor caches when > a request is not naturally aligned or is less than 4 bytes. This is > defensive change. I'm wondering if we should just document that this routine does not support unaligned transfers? Maybe backed by a debug mode that does the alignment check.
