On Fri, Oct 5, 2018 at 2:56 AM Jan Kara <jack@xxxxxxx> wrote: > > On Thu 04-10-18 21:28:14, Dan Williams wrote: > > On Thu, Oct 4, 2018 at 9:01 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > > > On Thu, Oct 4, 2018 at 7:52 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > On Thu, Oct 04, 2018 at 06:57:52PM -0700, Dan Williams wrote: > > > > > On Thu, Oct 4, 2018 at 9:27 AM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > > > > > On Thu 27-09-18 11:22:22, Dan Williams wrote: > > > > > > > On Thu, Sep 27, 2018 at 6:41 AM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > > > > > > > > > On Thu 27-09-18 06:28:43, Matthew Wilcox wrote: > > > > > > > > > On Thu, Sep 27, 2018 at 01:23:32PM +0200, Jan Kara wrote: > > > > > > > > > > When dax_lock_mapping_entry() has to sleep to obtain entry lock, it will > > > > > > > > > > fail to unlock mapping->i_pages spinlock and thus immediately deadlock > > > > > > > > > > against itself when retrying to grab the entry lock again. Fix the > > > > > > > > > > problem by unlocking mapping->i_pages before retrying. > > > > > > > > > > > > > > > > > > It seems weird that xfstests doesn't provoke this ... > > > > > > > > > > > > > > > > The function currently gets called only from mm/memory-failure.c. And yes, > > > > > > > > we are lacking DAX hwpoison error tests in fstests... > > > > > > > > > > > > > > I have an item on my backlog to port the ndctl unit test that does > > > > > > > memory_failure() injection vs ext4 over to fstests. That said I've > > > > > > > been investigating a deadlock on ext4 caused by this test. When I saw > > > > > > > this patch I hoped it was root cause, but the test is still failing > > > > > > > for me. Vishal is able to pass the test on his system, so the failure > > > > > > > mode is timing dependent. I'm running this patch on top of -rc5 and > > > > > > > still seeing the following deadlock. > > > > > > > > > > > > I went through the code but I don't see where the problem could be. How can > > > > > > I run that test? Is KVM enough or do I need hardware with AEP dimms? > > > > > > > > > > KVM is enough... however, I have found a hack that makes the test pass: > > > > > > > > > > diff --git a/mm/filemap.c b/mm/filemap.c > > > > > index 52517f28e6f4..d7f035b1846e 100644 > > > > > --- a/mm/filemap.c > > > > > +++ b/mm/filemap.c > > > > > @@ -1668,6 +1668,9 @@ unsigned find_get_entries(struct address_space *mapping, > > > > > goto repeat; > > > > > } > > > > > export: > > > > > + if (iter.index < start) > > > > > + continue; > > > > > + > > > > > indices[ret] = iter.index; > > > > > entries[ret] = page; > > > > > if (++ret == nr_entries) > > > > > > > > > > Is this a radix bug? I would never expect: > > > > > > > > > > radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) > > > > > > > > > > ...to return entries with an index < start. Without that change above > > > > > we loop forever because dax_layout_busy_page() can't make forward > > > > > progress. I'll dig into the radix code tomorrow, but maybe someone > > > > > else will be me to it overnight. > > > > > > > > If 'start' is within a 2MB entry, iter.index can absolutely be less > > > > than start. I forget exactly what the radix tree code does, but I think > > > > it returns index set to the canonical/base index of the entry. > > > > > > Ok, that makes sense. Then the bug is in dax_layout_busy_page() which > > > needs to increment 'index' by the entry size. This might also explain > > > why not every run sees it because you may get lucky and have a 4K > > > entry. > > > > Hmm, no 2MB entry here. > > > > We go through the first find_get_entries and export: > > > > export start: 0x0 index: 0x0 page: 0x822000a > > export start: 0x0 index: 0x200 page: 0xcc3801a > > > > Then dax_layout_busy_page sets 'start' to 0x201, and find_get_entries returns: > > > > export start: 0x201 index: 0x200 page: 0xcc3801a > > > > ...forevermore. > > Are you sure there's not 2MB entry starting at index 0x200? Because if > there was, we'd get infinite loop exactly as you describe in > dax_layout_busy_page() My debug code was buggy, these are 2MB entries, I'll send out a fix. > AFAICT. And it seems to me lot of other places > iterating over entries are borked in a similar way as they all assume that > doing +1 to the current index is guaranteeing them forward progress. Now > actual breakage resulting from this is limited as only DAX uses multiorder > entries and thus not many of these iterators actually ever get called for > radix tree with multiorder entries (e.g. tmpfs still inserts every 4k subpage > of THP into the radix tree and iteration functions usually handles > tail subpages in a special way). But this would really deserve larger > cleanup. Yeah, it's a subtle detail waiting to trip up new multi-order-radix users. The shift reported in the iterator is 6 in this case.
