On Sat, Aug 09, 2014 at 04:11:06PM -0700, Hugh Dickins wrote: > On Fri, 1 Aug 2014, Naoya Horiguchi wrote: > > > After fixing locking in follow_page(FOLL_GET) for hugepages, I start to > > observe the BUG of "get_page() on refcount 0 page" in hugetlb_fault() in > > the same test. > > > > I'm not exactly sure about how this race is triggered, but hugetlb_fault() > > calls pte_page() and get_page() outside page table lock, so it's not safe. > > This patch checks the refcount of the gotten page, and aborts the page fault > > if the refcount is 0, expecting to retry. > > > > Fixes: 66aebce747ea ("hugetlb: fix race condition in hugetlb_fault()") > > > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> > > Cc: <stable@xxxxxxxxxxxxxxx> # [3.12+] > > > I disagree with your 3.12+ annotation there: you may have hit the issue > in testing your hugepage migration work, but it's older than that: the > problematic get_page() was introduced in 3.4, and has been backported > to 3.2-stable: so 3.2+. Right, thanks. > I was suspicious of this patch at first, then on the point of giving it > an Ack, and then realized that I had been right to be suspicious of it. > > You're not the first the get the sequence wrong here; and it won't be > surprising if there are other instances of subtle get_page_unless_zero() > misuse elsewhere in the tree (I dare not look! someone else please do). > > It's not the use of get_page_unless_zero() itself that is wrong, it's > the unjustified confidence in it: what's wrong is the lock_page() after. > > As you have found, and acknowledged with get_page_unless_zero(), is > that the page here may be stale, it might be already freed, it might > be already reused. If reused, then its page_count will no longer be 0, > but the new user expects to have sole ownership of the page. The new > owner might be using __set_page_locked() (or one of the other nonatomic > flags operations), or "if (!trylock_page(newpage)) BUG()" like > migration's move_to_new_page(). > > We are dealing with a recently-hugetlb page here: that might make the > race I'm describing even less likely than with usual order:0 pages, > but I don't think it eliminates it. I agree. > What to do instead? The first answer that occurs to me is to move the > the pte_page,get_page down after the pte_same check inside the spin_lock, > and only then do trylock_page(), backing out to wait_on_page_locked and > retry or refault if not. I think that should work. According to the lock ordering commented in mm/rmap.c, page lock is prior to page table lock, so we can't take page lock inside page table lock. But with trylock_page() we check if the page lock is taken or not, so we can avoid deadlock. > Though if doing that, it might be more sensible only to trylock_page > before dropping ptl inside hugetlb_cow(). That would be a bigger, > maybe harder to backport, rearrangement. Yes, the patch will be somewhat complicated for stable, and we can't avoid that. Thanks, Naoya Horiguchi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>