Re: [PATCH v2] mm, memcg: fix reclaim deadlock with writeback

[Liu Bo]

On Wed, Dec 12, 2018 at 04:50:55PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Liu Bo has experienced a deadlock between memcg (legacy) reclaim and the
> ext4 writeback
> task1:
> [<ffffffff811aaa52>] wait_on_page_bit+0x82/0xa0
> [<ffffffff811c5777>] shrink_page_list+0x907/0x960
> [<ffffffff811c6027>] shrink_inactive_list+0x2c7/0x680
> [<ffffffff811c6ba4>] shrink_node_memcg+0x404/0x830
> [<ffffffff811c70a8>] shrink_node+0xd8/0x300
> [<ffffffff811c73dd>] do_try_to_free_pages+0x10d/0x330
> [<ffffffff811c7865>] try_to_free_mem_cgroup_pages+0xd5/0x1b0
> [<ffffffff8122df2d>] try_charge+0x14d/0x720
> [<ffffffff812320cc>] memcg_kmem_charge_memcg+0x3c/0xa0
> [<ffffffff812321ae>] memcg_kmem_charge+0x7e/0xd0
> [<ffffffff811b68a8>] __alloc_pages_nodemask+0x178/0x260
> [<ffffffff8120bff5>] alloc_pages_current+0x95/0x140
> [<ffffffff81074247>] pte_alloc_one+0x17/0x40
> [<ffffffff811e34de>] __pte_alloc+0x1e/0x110
> [<ffffffffa06739de>] alloc_set_pte+0x5fe/0xc20
> [<ffffffff811e5d93>] do_fault+0x103/0x970
> [<ffffffff811e6e5e>] handle_mm_fault+0x61e/0xd10
> [<ffffffff8106ea02>] __do_page_fault+0x252/0x4d0
> [<ffffffff8106ecb0>] do_page_fault+0x30/0x80
> [<ffffffff8171bce8>] page_fault+0x28/0x30
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> task2:
> [<ffffffff811aadc6>] __lock_page+0x86/0xa0
> [<ffffffffa02f1e47>] mpage_prepare_extent_to_map+0x2e7/0x310 [ext4]
> [<ffffffffa08a2689>] ext4_writepages+0x479/0xd60
> [<ffffffff811bbede>] do_writepages+0x1e/0x30
> [<ffffffff812725e5>] __writeback_single_inode+0x45/0x320
> [<ffffffff81272de2>] writeback_sb_inodes+0x272/0x600
> [<ffffffff81273202>] __writeback_inodes_wb+0x92/0xc0
> [<ffffffff81273568>] wb_writeback+0x268/0x300
> [<ffffffff81273d24>] wb_workfn+0xb4/0x390
> [<ffffffff810a2f19>] process_one_work+0x189/0x420
> [<ffffffff810a31fe>] worker_thread+0x4e/0x4b0
> [<ffffffff810a9786>] kthread+0xe6/0x100
> [<ffffffff8171a9a1>] ret_from_fork+0x41/0x50
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> He adds
> : task1 is waiting for the PageWriteback bit of the page that task2 has
> : collected in mpd->io_submit->io_bio, and tasks2 is waiting for the LOCKED
> : bit the page which tasks1 has locked.
> 
> More precisely task1 is handling a page fault and it has a page locked
> while it charges a new page table to a memcg. That in turn hits a memory
> limit reclaim and the memcg reclaim for legacy controller is waiting on
> the writeback but that is never going to finish because the writeback
> itself is waiting for the page locked in the #PF path. So this is
> essentially ABBA deadlock.

Thanks for the patch, Michal.

Could you please append the followings (quoted from your reply in
other thread)?  It'd be much easier for reviewers to pick up what was
happening.

-----------------------------------------------------------------
                                        lock_page(B)
                                        SetPageWriteback(B)
                                        unlock_page(B)
lock_page(A)
                                        lock_page(A)
pte_alloc_pne
  shrink_page_list
    wait_on_page_writeback(B)
                                        SetPageWriteback(A)
                                        unlock_page(A)

                                        # flush A, B to clear the writeback 
-----------------------------------------------------------------

thanks,
-liubo

> 
> Waiting for the writeback in legacy memcg controller is a workaround
> for pre-mature OOM killer invocations because there is no dirty IO
> throttling available for the controller. There is no easy way around
> that unfortunately. Therefore fix this specific issue by pre-allocating
> the page table outside of the page lock. We have that handy
> infrastructure for that already so simply reuse the fault-around pattern
> which already does this.
> 
> Reported-and-Debugged-by: Liu Bo <bo.liu@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/memory.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 4ad2d293ddc2..bb78e90a9b70 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2993,6 +2993,17 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	vm_fault_t ret;
>  
> +	/*
> +	 * Preallocate pte before we take page_lock because this might lead to
> +	 * deadlocks for memcg reclaim which waits for pages under writeback.
> +	 */
> +	if (pmd_none(*vmf->pmd) && !vmf->prealloc_pte) {
> +		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm, vmf->address);
> +		if (!vmf->prealloc_pte)
> +			return VM_FAULT_OOM;
> +		smp_wmb(); /* See comment in __pte_alloc() */
> +	}
> +
>  	ret = vma->vm_ops->fault(vmf);
>  	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
>  			    VM_FAULT_DONE_COW)))
> -- 
> 2.19.2