Messages in this thread | | | Date | Wed, 8 Jan 2014 17:13:38 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new_page() race |
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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. 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.
> > For example, __get_page_tail() roughly does: > > > > // PageTail(page) was already checked > > > > page_head = page->first_page; > > > > /* WINDOW */ > > > > get_page_unless_zero(page_head); > > > > compound_lock(page_head); > > > > recheck PageTail(page) to ensure page_head is still valid > > > > However, in the WINDOW above, split_huge_page() can split this huge page. > > After that its head can be freed and reallocated. Of course, I don't think > > it is possible to hit this race in practice, but still this looks wrong. > > > > I can't think of a reason why we would actually hit that race in practice
Agreed, the window is tiny, unlikely this possible.
> I do not think we > should stick a write barrier into the page allocator fast path.
OK, I won't argue, I leave this to you and Andrea.
But I still think this code needs other cleanups/simplifications. In particular get_futex_key()->__get_user_pages_fast() should die imho.
Oleg.
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