On Thu, Jun 10, 2021 at 10:18:25AM +1000, Alistair Popple wrote: > > > The main problem is split_huge_pmd_address() unconditionally calls a mmu > > > notifier so I would need to plumb in passing an owner everywhere which could > > > get messy. > > > > Could I ask why? split_huge_pmd_address() will notify with CLEAR, so I'm a bit > > confused why we need to pass over the owner. > > Sure, it is the same reason we need to pass it for the exclusive notifier. > Any invalidation during the make exclusive operation will break the mmu read > side critical section forcing a retry of the operation. The owner field is what > is used to filter out invalidations (such as the exclusive invalidation) that > don't need to be retried. Do you mean the mmu_interval_read_begin|retry() calls? Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit call split_huge_pmd_address(), afaict. Since both of them use __split_huge_pmd() internally which will generate that unwanted CLEAR notify. If that's the case, I think it fails because split_huge_pmd_address() will trigger that CLEAR notify unconditionally (even if it's not a thp; not sure whether it should be optimized to not notify at all... definitely another story), while FOLL_SPLIT_PMD will skip the notify as it calls split_huge_pmd() instead, who checks the pmd before calling __split_huge_pmd(). Does it also mean that if there's a real THP it won't really work? As then FOLL_SPLIT_PMD will start to trigger that CLEAR notify too, I think.. -- Peter Xu
