On Mon 29-07-13 17:24:53, Minchan Kim wrote: > Hi Michal, > > On Thu, Jul 25, 2013 at 03:30:40PM +0200, Michal Hocko wrote: > > On Wed 24-07-13 11:44:28, Minchan Kim wrote: > > > On Tue, Jul 23, 2013 at 04:01:20PM +0200, Michal Hocko wrote: > > > > On Fri 19-07-13 09:13:03, Minchan Kim wrote: > > > > > On Thu, Jul 18, 2013 at 11:12:24PM +0530, Aneesh Kumar K.V wrote: > > > > [...] > > > > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > > > > > > index 83aff0a..2cb1be3 100644 > > > > > > --- a/mm/hugetlb.c > > > > > > +++ b/mm/hugetlb.c > > > > > > @@ -3266,8 +3266,8 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud) > > > > > > put_page(virt_to_page(spte)); > > > > > > spin_unlock(&mm->page_table_lock); > > > > > > out: > > > > > > - pte = (pte_t *)pmd_alloc(mm, pud, addr); > > > > > > mutex_unlock(&mapping->i_mmap_mutex); > > > > > > + pte = (pte_t *)pmd_alloc(mm, pud, addr); > > > > > > return pte; > > > > > > > > > > I am blind on hugetlb but not sure it doesn't break eb48c071. > > > > > Michal? > > > > > > > > Well, it is some time since I debugged the race and all the details > > > > vanished in the meantime. But this part of the changelog suggests that > > > > this indeed breaks the fix: > > > > " > > > > This patch addresses the issue by moving pmd_alloc into huge_pmd_share > > > > which guarantees that the shared pud is populated in the same critical > > > > section as pmd. This also means that huge_pte_offset test in > > > > huge_pmd_share is serialized correctly now which in turn means that the > > > > success of the sharing will be higher as the racing tasks see the pud > > > > and pmd populated together. > > > > " > > > > > > > > Besides that I fail to see how moving pmd_alloc down changes anything. > > > > Even if pmd_alloc triggered reclaim then we cannot trip over the same > > > > i_mmap_mutex as hugetlb pages are not reclaimable because they are not > > > > on the LRU. > > > > > > I thought we could map some part of binary with normal page and other part > > > of the one with MAP_HUGETLB so that a address space could have both normal > > > page and HugeTLB page. Okay, it's impossible so HugeTLB pages are not on LRU. > > > Then, above lockdep warning is totally false positive. > > > Best solution is avoiding pmd_alloc with holding i_mmap_mutex but we need it > > > to fix eb48c071 so how about this if we couldn't have a better idea? > > > > Shouldn't we rather use a hugetlb specific lock_class_key. I am not > > familiar with lockdep much but something like bellow should do the > > trick? > > Looks good to me. > Could you resend it with formal patch with Ccing Peter for Dave to confirm it? > Below just a nitpick. I would have posted it already but I have to confess I am really not familiar with lockdep and what is the good way to fix such a false positive. Peter, for you context the lockdep splat has been reported here: https://lkml.org/lkml/2013/7/17/381 Minchan has proposed to workaround it by using SINGLE_DEPTH_NESTING https://lkml.org/lkml/2013/7/23/812 my idea was to use a separate class key for hugetlb as it is quite special in many ways: https://lkml.org/lkml/2013/7/25/277 What is the preferred way of fixing such an issue? Thanks! -- Michal Hocko SUSE Labs -- 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>