Messages in this thread | | | Date | Wed, 8 Jan 2014 18:02:02 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new_page() race |
| |
On Wed, Jan 08, 2014 at 05:13:38PM +0100, Oleg Nesterov wrote: > On 01/08, Mel Gorman wrote: > > > > On Sat, Jan 04, 2014 at 05:43:47PM +0100, Oleg Nesterov wrote: > > > > > > get/put_page(thp_tail) paths do get_page_unless_zero(page_head) + > > > compound_lock(). In theory this page_head can be already freed and > > > reallocated as alloc_pages(__GFP_COMP, smaller_order). In this case > > > get_page_unless_zero() can succeed right after set_page_refcounted(), > > > and compound_lock() can race with the non-atomic __SetPageHead() in > > > prep_compound_page(). > > > > > This patch is putting a write barrier in the page allocator fast path and > > that is going to be a leading cause of Sad Face. We already have seen > > large regressions before when write barriers were introduced to the page > > allocator paths for cpusets. Sticking it under CONFIG_TRANSPARENT_HUGEPAGE > > does not really address the issue. > > As you already mentioned in another email, smp_wmb() is mostly nop. On > x86_64 at least.
Which sometimes means that it'll just take longer for someone to find it and bitch about it.
> Although perhaps it would be nice to have > > static inline void atomic_store_release(atomic_t *v, int i) > { > smp_store_release(&v->counter, i); > } > > > > Yes, but thp can access this page_head via stale pointer, tail->first_page, > > > if it races with split_huge_page_refcount(). > > > > To justify the introduction of a performance regression we need to be 100% > > sure this race actually exists > > See below. But let me remind that I never looked at this code before, > I can be easily wrong. > > > and not just theoretical. > > It is theoretical anyway, I guess. > > > For futex, the THP page (and the tail) must have been discovered via > > the page tables in which case the page tables are temporarily preventing > > the page being freed to the allocator. > > Yes. But, for example, get_futex_key() does > > if (unlikely(PageTail(page))) { > put_page(page); > > why this put_page() can't race with _split? If nothing else, another thread > can unmap the part of this vma. >
The race is not prevented but that does not mean it matters. Basic scenario where a split starts after the PageTail check but before the put_page in get_futex_key
CPU A get_futex_key -> fast gup, page table removing prevents parallel unmap and free -> gup_huge_pmd (arch/x86/mm/gup.c at least) -> get_huge_page_tail (increment page tail _map_count) -> get_huge_page_multiple (increment ref on head page) -> Check PageTail CPU B split_huge_page_to_list -> split_huge_page_refcount spin_lock_irq(lru_lock) compound_lock -> put_page(tail_page) ->put_compound_page looks up head page takes reference unless zero compound_lock (block) complete split compound_unlock check PageTail
This put_page blocks on the compound lock, finds the page is no longer a PageTail as the split barriers correctly and backs out gracefully. So sure, splits can race but the case is cared for.
The parallel unmap is prevented by get_huge_page_multiple in the gup path and held in place until put_page_compound frees it later.
I still don't see the case where a page gets freed to the page allocator that causes weird problems here. Unfortunately, I also recognise I have tunnel vision because subconsciously I don't *want* to see a problem here that justifies adding a write barrier. Andrea may stomp all over this reasoning in which case we'll get a good comment for the smp_wmb :/
-- Mel Gorman SUSE Labs
| |