On Tue, Jun 16, 2020 at 5:27 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Tue 16-06-20 17:05:25, Yafang Shao wrote:
> > On Tue, Jun 16, 2020 at 4:16 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > On Mon 15-06-20 07:56:21, Yafang Shao wrote:
> > > > Recently there is a XFS deadlock on our server with an old kernel.
> > > > This deadlock is caused by allocating memory in xfs_map_blocks() while
> > > > doing writeback on behalf of memroy reclaim. Although this deadlock happens
> > > > on an old kernel, I think it could happen on the upstream as well. This
> > > > issue only happens once and can't be reproduced, so I haven't tried to
> > > > reproduce it on upsteam kernel.
> > > >
> > > > Bellow is the call trace of this deadlock.
> > > > [480594.790087] INFO: task redis-server:16212 blocked for more than 120 seconds.
> > > > [480594.790087] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > > [480594.790088] redis-server D ffffffff8168bd60 0 16212 14347 0x00000004
> > > > [480594.790090] ffff880da128f070 0000000000000082 ffff880f94a2eeb0 ffff880da128ffd8
> > > > [480594.790092] ffff880da128ffd8 ffff880da128ffd8 ffff880f94a2eeb0 ffff88103f9d6c40
> > > > [480594.790094] 0000000000000000 7fffffffffffffff ffff88207ffc0ee8 ffffffff8168bd60
> > > > [480594.790096] Call Trace:
> > > > [480594.790101] [<ffffffff8168dce9>] schedule+0x29/0x70
> > > > [480594.790103] [<ffffffff8168b749>] schedule_timeout+0x239/0x2c0
> > > > [480594.790111] [<ffffffff8168d28e>] io_schedule_timeout+0xae/0x130
> > > > [480594.790114] [<ffffffff8168d328>] io_schedule+0x18/0x20
> > > > [480594.790116] [<ffffffff8168bd71>] bit_wait_io+0x11/0x50
> > > > [480594.790118] [<ffffffff8168b895>] __wait_on_bit+0x65/0x90
> > > > [480594.790121] [<ffffffff811814e1>] wait_on_page_bit+0x81/0xa0
> > > > [480594.790125] [<ffffffff81196ad2>] shrink_page_list+0x6d2/0xaf0
> > > > [480594.790130] [<ffffffff811975a3>] shrink_inactive_list+0x223/0x710
> > > > [480594.790135] [<ffffffff81198225>] shrink_lruvec+0x3b5/0x810
> > > > [480594.790139] [<ffffffff8119873a>] shrink_zone+0xba/0x1e0
> > > > [480594.790141] [<ffffffff81198c20>] do_try_to_free_pages+0x100/0x510
> > > > [480594.790143] [<ffffffff8119928d>] try_to_free_mem_cgroup_pages+0xdd/0x170
> > > > [480594.790145] [<ffffffff811f32de>] mem_cgroup_reclaim+0x4e/0x120
> > > > [480594.790147] [<ffffffff811f37cc>] __mem_cgroup_try_charge+0x41c/0x670
> > > > [480594.790153] [<ffffffff811f5cb6>] __memcg_kmem_newpage_charge+0xf6/0x180
> > > > [480594.790157] [<ffffffff8118c72d>] __alloc_pages_nodemask+0x22d/0x420
> > > > [480594.790162] [<ffffffff811d0c7a>] alloc_pages_current+0xaa/0x170
> > > > [480594.790165] [<ffffffff811db8fc>] new_slab+0x30c/0x320
> > > > [480594.790168] [<ffffffff811dd17c>] ___slab_alloc+0x3ac/0x4f0
> > > > [480594.790204] [<ffffffff81685656>] __slab_alloc+0x40/0x5c
> > > > [480594.790206] [<ffffffff811dfc43>] kmem_cache_alloc+0x193/0x1e0
> > > > [480594.790233] [<ffffffffa04fab67>] kmem_zone_alloc+0x97/0x130 [xfs]
> > > > [480594.790247] [<ffffffffa04f90ba>] _xfs_trans_alloc+0x3a/0xa0 [xfs]
> > > > [480594.790261] [<ffffffffa04f915c>] xfs_trans_alloc+0x3c/0x50 [xfs]
> > >
> > > Now with a more explanation from Dave I have got back to the original
> > > backtrace. Not sure which kernel version you are using but this path
> > > should have passed xfs_trans_reserve which sets PF_MEMALLOC_NOFS and
> > > this in turn should have made __alloc_pages_nodemask to use __GFP_NOFS
> > > and the memcg reclaim shouldn't ever wait_on_page_writeback (pressumably
> > > this is what the io_schedule is coming from).
> >
> > Hi Michal,
> >
> > The page is allocated before calling xfs_trans_reserve, so the
> > PF_MEMALLOC_NOFS hasn't been set yet.
> > See bellow,
> >
> > xfs_trans_alloc
> > kmem_zone_zalloc() <<< GPF_NOFS hasn't been set yet, but it may
> > trigger memory reclaim
> > xfs_trans_reserve() <<<< GPF_NOFS is set here (for the kernel
> > prior to commit 9070733b4efac, it is PF_FSTRANS)
>
> You are right, I have misread the actual allocation side. 8683edb7755b8
> has added KM_NOFS and 73d30d48749f8 has removed it. I cannot really
> judge the correctness here.
>
The history is complicated, but it doesn't matter.
Let's turn back to the upstream kernel now. As I explained in the commit log,
xfs_vm_writepages
-> iomap_writepages.
-> write_cache_pages
-> lock_page <<<< This page is locked.
-> writepages ( which is iomap_do_writepage)
-> xfs_map_blocks
-> xfs_convert_blocks
-> xfs_bmapi_convert_delalloc
-> xfs_trans_alloc
-> kmem_zone_zalloc //It should alloc page
with GFP_NOFS
If GFP_NOFS isn't set in xfs_trans_alloc(), the kmem_zone_zalloc() may
trigger the memory reclaim then it may wait on the page locked in
write_cache_pages() ...
That means the ->writepages should be set with GFP_NOFS to avoid this
recursive filesystem reclaim.
--
Thanks
Yafang
