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

[Michal Hocko]

On Wed 12-12-18 14:58:37, Kirill A. Shutemov wrote:
> On Tue, Dec 11, 2018 at 02:26:45PM +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.
> > 
> > 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>
> > ---
> > Hi,
> > this has been originally reported here [1]. While it could get worked
> > around in the fs, catching the allocation early sounds like a preferable
> > approach. Liu Bo has noted that he is not able to reproduce this anymore
> > because kmem accounting has been disabled in their workload but this
> > should be quite straightforward to review.
> > 
> > There are probably other hidden __GFP_ACCOUNT | GFP_KERNEL allocations
> > from under a fs page locked but they should be really rare. I am not
> > aware of a better solution unfortunately.
> 
> Okay, I have spent some time on the issue and was not able to find a
> better solution too. But I cannot say I like it.

Ohh, I do not like it either. I can make it more targeted by abstracting
sane_reclaim() and using it for the check but I am not sure this is
really more helpful.
 
> I think we need to spend more time on making ->prealloc_pte useful: looks
> like it would help to convert vmf_insert_* helpers to take struct vm_fault
> * as input and propagate it down to pmd population point. Otherwise DAX
> and drivers would alloacate the page table for nothing.

Yes this would be an obvious enahancement.

> Have you considered if we need anything similar for anon path? Is it
> possible to have similar deadlock with swaping rather than writeback?

No, I do not see anon path to allocated under page lock.
-- 
Michal Hocko
SUSE Labs