On Tue 17-01-17 15:37:05, Vishal Verma wrote: > I do mean that in the filesystem, for every IO, the badblocks will be > checked. Currently, the pmem driver does this, and the hope is that the > filesystem can do a better job at it. The driver unconditionally checks > every IO for badblocks on the whole device. Depending on how the > badblocks are represented in the filesystem, we might be able to quickly > tell if a file/range has existing badblocks, and error out the IO > accordingly. > > At mount the the fs would read the existing badblocks on the block > device, and build its own representation of them. Then during normal > use, if the underlying badblocks change, the fs would get a notification > that would allow it to also update its own representation. So I believe we have to distinguish three cases so that we are on the same page. 1) PMEM is exposed only via a block interface for legacy filesystems to use. Here, all the bad blocks handling IMO must happen in NVDIMM driver. Looking from outside, the IO either returns with EIO or succeeds. As a result you cannot ever ger rid of bad blocks handling in the NVDIMM driver. 2) PMEM is exposed for DAX aware filesystem. This seems to be what you are mostly interested in. We could possibly do something more efficient than what NVDIMM driver does however the complexity would be relatively high and frankly I'm far from convinced this is really worth it. If there are so many badblocks this would matter, the HW has IMHO bigger problems than performance. 3) PMEM filesystem - there things are even more difficult as was already noted elsewhere in the thread. But for now I'd like to leave those aside not to complicate things too much. Now my question: Why do we bother with badblocks at all? In cases 1) and 2) if the platform can recover from MCE, we can just always access persistent memory using memcpy_mcsafe(), if that fails, return -EIO. Actually that seems to already happen so we just need to make sure all places handle returned errors properly (e.g. fs/dax.c does not seem to) and we are done. No need for bad blocks list at all, no slow down unless we hit a bad cell and in that case who cares about performance when the data is gone... For platforms that cannot recover from MCE - just buy better hardware ;). Seriously, I have doubts people can seriously use a machine that will unavoidably randomly reboot (as there is always a risk you hit error that has not been uncovered by background scrub). But maybe for big cloud providers the cost savings may offset for the inconvenience, I don't know. But still for that case a bad blocks handling in NVDIMM code like we do now looks good enough? Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
