On Thu 27-10-16 13:54:49, Ross Zwisler wrote: > On Thu, Oct 27, 2016 at 09:07:50PM +0200, Jan Kara wrote: > > When testing my DAX patches rebased on top of Ross' DAX PMD series, I've > > come across the following issue with generic/344 test from xfstests. The > > test ends in an infinite fault loop when we fault index 0 over and over > > again never finishing the fault. The problem is that we do a write fault > > for index 0 when there is PMD for that index. So we enter wp_huge_pmd(). > > For whatever reason that returns VM_FAULT_FALLBACK so we continue to > > handle_pte_fault(). There we do > > > > if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd)) > > > > check which is true - the PMD we have is pmd_trans_huge() - so we 'return > > 0' and that results in retrying the fault and all happens from the > > beginning again. > > > > It isn't quite obvious how to break that cycle to me. The comment before > > pmd_none_or_trans_huge_or_clear_bad() goes to great lengths explaining > > possible races when PMD is pmd_trans_huge() so it needs careful evaluation > > what needs to be done for DAX. Ross, any idea? > > I'll try & reproduce this, and I'll get back to you. For me it happened with ext4 which returned VM_FAULT_FALLBACK from its pmd_fault handler on write fault (likely we were not able to allocate sufficiently large contiguous hunk). So I'm not sure you will be able to easily reproduce just with your series. However tweaking XFS to return VM_FAULT_FALLBACK when FAULT_FLAG_WRITE is set should do the trick. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
