On Wed, Nov 21, 2018 at 08:35:28AM +0800, Yang Shi wrote:
>
>
> On 11/20/18 9:42 PM, Kirill A. Shutemov wrote:
> > On Tue, Nov 20, 2018 at 08:10:51PM +0800, Yang Shi wrote:
> > >
> > > On 11/20/18 4:57 PM, Kirill A. Shutemov wrote:
> > > > On Fri, Nov 16, 2018 at 08:56:04AM -0800, Yang Shi wrote:
> > > > > > a8dda165ec vfree: add debug might_sleep()
> > > > > > dd2283f260 mm: mmap: zap pages with read mmap_sem in munmap
> > > > > > 5929a1f0ff Merge tag 'riscv-for-linus-4.20-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/palmer/riscv-linux
> > > > > > 0bc80e3cb0 Add linux-next specific files for 20181114
> > > > > > +-----------------------------------------------------+------------+------------+------------+---------------+
> > > > > > | | a8dda165ec | dd2283f260 | 5929a1f0ff | next-20181114 |
> > > > > > +-----------------------------------------------------+------------+------------+------------+---------------+
> > > > > > | boot_successes | 314 | 178 | 190 | 168 |
> > > > > > | boot_failures | 393 | 27 | 21 | 40 |
> > > > > > | WARNING:held_lock_freed | 383 | 23 | 17 | 39 |
> > > > > > | is_freeing_memory#-#,with_a_lock_still_held_there | 383 | 23 | 17 | 39 |
> > > > > > | BUG:unable_to_handle_kernel | 5 | 2 | 4 | 1 |
> > > > > > | Oops:#[##] | 9 | 3 | 4 | 1 |
> > > > > > | EIP:debug_check_no_locks_freed | 9 | 3 | 4 | 1 |
> > > > > > | Kernel_panic-not_syncing:Fatal_exception | 9 | 3 | 4 | 1 |
> > > > > > | Mem-Info | 4 | 1 | | |
> > > > > > | invoked_oom-killer:gfp_mask=0x | 1 | 1 | | |
> > > > > > | WARNING:at_kernel/locking/lockdep.c:#lock_downgrade | 0 | 6 | 4 | 7 |
> > > > > > | EIP:lock_downgrade | 0 | 6 | 4 | 7 |
> > > > > > +-----------------------------------------------------+------------+------------+------------+---------------+
> > > > > >
> > > > > > [ 96.288009] random: get_random_u32 called from arch_rnd+0x3c/0x70 with crng_init=0
> > > > > > [ 96.359626] input_id (331) used greatest stack depth: 6360 bytes left
> > > > > > [ 96.749228] grep (358) used greatest stack depth: 6336 bytes left
> > > > > > [ 96.921470] network.sh (341) used greatest stack depth: 6212 bytes left
> > > > > > [ 97.262340]
> > > > > > [ 97.262587] =========================
> > > > > > [ 97.263072] WARNING: held lock freed!
> > > > > > [ 97.263536] 4.19.0-06969-gdd2283f #1 Not tainted
> > > > > > [ 97.264110] -------------------------
> > > > > > [ 97.264575] udevd/198 is freeing memory 9c16c930-9c16c99b, with a lock still held there!
> > > > > > [ 97.265542] (ptrval) (&anon_vma->rwsem){....}, at: unlink_anon_vmas+0x14e/0x420
> > > > > > [ 97.266450] 1 lock held by udevd/198:
> > > > > > [ 97.266924] #0: (ptrval) (&mm->mmap_sem){....}, at: __do_munmap+0x531/0x730
> > > > > I have not figured out what this is caused by. But, the below warning looks
> > > > > more confusing. This might be caused by the below one.
> > > > I *think* we need to understand more about what detached VMAs mean for
> > > > rmap. The anon_vma for these VMAs still reachable for the rmap and
> > > > therefore VMA too. I don't quite grasp what is implications of this, but
> > > > it doesn't look good.
> > > I'm supposed before accessing anon_vma, VMA need to be found by find_vma()
> > > first, right? But, finding VMA need hold mmap_sem, once detach VMAs is
> > > called, others should not be able to find the VMAs anymore. So, the anon_vma
> > > should not be reachable except the munmap caller.
> > No. anon_vma can be reached from page->mapping. The page can be reached
> > during physcal memory scan or if the page is shared (across fork()). None
> > of these accesses require mmap_sem.
>
> If they don't require mmap_sem at all, this problem should be valid
> regardless of the optimization. We just downgraded write mmap_sem to read,
> but still hold it.
I tend to agree with you.
But having the crash in the picture makes me wounder if there's scenario
when VMA reachable via anon_vma, but not via find_vma() causes a problem.
I cannot think of any, but who knows.
--
Kirill A. Shutemov
