On Friday, 11 June 2021 4:04:35 AM AEST Peter Xu wrote: > External email: Use caution opening links or attachments > > > 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? Yep. > 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. Agree that gup calls __split_huge_pmd() via split_huge_pmd_address() which will always CLEAR. However gup only calls split_huge_pmd_address() if it finds a thp pmd. In follow_pmd_mask() we have: if (likely(!pmd_trans_huge(pmdval))) return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); So I don't think we have a problem here. > 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 >
