Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

[Qian Cai <cai@lca.pw>]

On Wed, 2019-10-09 at 16:34 +0200, Michal Hocko wrote:
> On Wed 09-10-19 10:19:44, Qian Cai wrote:
> > On Wed, 2019-10-09 at 15:51 +0200, Michal Hocko wrote:
> 
> [...]
> > > Can you paste the full lock chain graph to be sure we are on the same
> > > page?
> > 
> > WARNING: possible circular locking dependency detected
> > 5.3.0-next-20190917 #8 Not tainted
> > ------------------------------------------------------
> > test.sh/8653 is trying to acquire lock:
> > ffffffff865a4460 (console_owner){-.-.}, at:
> > console_unlock+0x207/0x750
> > 
> > but task is already holding lock:
> > ffff88883fff3c58 (&(&zone->lock)->rlock){-.-.}, at:
> > __offline_isolated_pages+0x179/0x3e0
> > 
> > which lock already depends on the new lock.
> > 
> > 
> > the existing dependency chain (in reverse order) is:
> > 
> > -> #3 (&(&zone->lock)->rlock){-.-.}:
> >        __lock_acquire+0x5b3/0xb40
> >        lock_acquire+0x126/0x280
> >        _raw_spin_lock+0x2f/0x40
> >        rmqueue_bulk.constprop.21+0xb6/0x1160
> >        get_page_from_freelist+0x898/0x22c0
> >        __alloc_pages_nodemask+0x2f3/0x1cd0
> >        alloc_pages_current+0x9c/0x110
> >        allocate_slab+0x4c6/0x19c0
> >        new_slab+0x46/0x70
> >        ___slab_alloc+0x58b/0x960
> >        __slab_alloc+0x43/0x70
> >        __kmalloc+0x3ad/0x4b0
> >        __tty_buffer_request_room+0x100/0x250
> >        tty_insert_flip_string_fixed_flag+0x67/0x110
> >        pty_write+0xa2/0xf0
> >        n_tty_write+0x36b/0x7b0
> >        tty_write+0x284/0x4c0
> >        __vfs_write+0x50/0xa0
> >        vfs_write+0x105/0x290
> >        redirected_tty_write+0x6a/0xc0
> >        do_iter_write+0x248/0x2a0
> >        vfs_writev+0x106/0x1e0
> >        do_writev+0xd4/0x180
> >        __x64_sys_writev+0x45/0x50
> >        do_syscall_64+0xcc/0x76c
> >        entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> This one looks indeed legit. pty_write is allocating memory from inside
> the port->lock. But this seems to be quite broken, right? The forward
> progress depends on GFP_ATOMIC allocation which might fail easily under
> memory pressure. So the preferred way to fix this should be to change
> the allocation scheme to use the preallocated buffer and size it from a
> context when it doesn't hold internal locks. It might be a more complex
> fix than using printk_deferred or other games but addressing that would
> make the pty code more robust as well.

I am not really sure if doing a surgery in pty code is better than fixing the
memory offline side as a short-term fix.