On Wed, Sep 25, 2019 at 10:25:30AM +0200, Peter Zijlstra wrote: > On Tue, Sep 24, 2019 at 05:24:58PM -0600, Yu Zhao wrote: > > We don't want to expose a non-hugetlb page to the fast gup running > > on a remote CPU before all local non-atomic ops on the page flags > > are visible first. > > > > For an anon page that isn't in swap cache, we need to make sure all > > prior non-atomic ops, especially __SetPageSwapBacked() in > > page_add_new_anon_rmap(), are ordered before set_pte_at() to prevent > > the following race: > > > > CPU 1 CPU1 > > set_pte_at() get_user_pages_fast() > > page_add_new_anon_rmap() gup_pte_range() > > __SetPageSwapBacked() SetPageReferenced() > > > > This demonstrates a non-fatal scenario. Though haven't been directly > > observed, the fatal ones can exist, e.g., PG_lock set by fast gup > > caller and then overwritten by __SetPageSwapBacked(). > > > > For an anon page that is already in swap cache or a file page, we > > don't need smp_wmb() before set_pte_at() because adding to swap or > > file cach serves as a valid write barrier. Using non-atomic ops > > thereafter is a bug, obviously. > > > > smp_wmb() is added following 11 of total 12 page_add_new_anon_rmap() > > call sites, with the only exception being > > do_huge_pmd_wp_page_fallback() because of an existing smp_wmb(). > > > > I'm thinking this patch make stuff rather fragile.. Should we instead > stick the barrier in set_p*d_at() instead? Or rather, make that store a > store-release?
I prefer it this way too, but I suspected the majority would be concerned with the performance implications, especially those looping set_pte_at()s in mm/huge_memory.c. If we have a consensus that smp_wmb() would be better off wrapped together with set_p*d_at() in a macro, I'd be glad to make the change. And it seems to me applying smp_store_release() would have to happen in each arch, which might be just as fragile.
