Message-ID: <2ab4b33e-f570-a6ff-6315-7d5a4614a7bd@google.com>
Date:   Mon, 20 Feb 2023 18:48:38 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Bharata B Rao <bharata@....com>,
        Alistair Popple <apopple@...dia.com>,
        Xin Hao <xhao@...ux.alibaba.com>,
        Minchan Kim <minchan@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        "Xu, Pengfei" <pengfei.xu@...el.com>,
        Christoph Hellwig <hch@....de>,
        Stefan Roesch <shr@...kernel.io>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH -v5 0/9] migrate_pages(): batch TLB flushing

On Mon, 20 Feb 2023, Huang, Ying wrote:

> Hi, Hugh,
> 
> Hugh Dickins <hughd@...gle.com> writes:
> 
> > On Mon, 13 Feb 2023, Huang Ying wrote:
> >
> >> From: "Huang, Ying" <ying.huang@...el.com>
> >> 
> >> Now, migrate_pages() migrate folios one by one, like the fake code as
> >> follows,
> >> 
> >>   for each folio
> >>     unmap
> >>     flush TLB
> >>     copy
> >>     restore map
> >> 
> >> If multiple folios are passed to migrate_pages(), there are
> >> opportunities to batch the TLB flushing and copying.  That is, we can
> >> change the code to something as follows,
> >> 
> >>   for each folio
> >>     unmap
> >>   for each folio
> >>     flush TLB
> >>   for each folio
> >>     copy
> >>   for each folio
> >>     restore map
> >> 
> >> The total number of TLB flushing IPI can be reduced considerably.  And
> >> we may use some hardware accelerator such as DSA to accelerate the
> >> folio copying.
> >> 
> >> So in this patch, we refactor the migrate_pages() implementation and
> >> implement the TLB flushing batching.  Base on this, hardware
> >> accelerated folio copying can be implemented.
> >> 
> >> If too many folios are passed to migrate_pages(), in the naive batched
> >> implementation, we may unmap too many folios at the same time.  The
> >> possibility for a task to wait for the migrated folios to be mapped
> >> again increases.  So the latency may be hurt.  To deal with this
> >> issue, the max number of folios be unmapped in batch is restricted to
> >> no more than HPAGE_PMD_NR in the unit of page.  That is, the influence
> >> is at the same level of THP migration.
> >> 
> >> We use the following test to measure the performance impact of the
> >> patchset,
> >> 
> >> On a 2-socket Intel server,
> >> 
> >>  - Run pmbench memory accessing benchmark
> >> 
> >>  - Run `migratepages` to migrate pages of pmbench between node 0 and
> >>    node 1 back and forth.
> >> 
> >> With the patch, the TLB flushing IPI reduces 99.1% during the test and
> >> the number of pages migrated successfully per second increases 291.7%.
> >> 
> >> Xin Hao helped to test the patchset on an ARM64 server with 128 cores,
> >> 2 NUMA nodes.  Test results show that the page migration performance
> >> increases up to 78%.
> >> 
> >> This patchset is based on mm-unstable 2023-02-10.
> >
> > And back in linux-next this week: I tried next-20230217 overnight.
> >
> > There is a deadlock in this patchset (and in previous versions: sorry
> > it's taken me so long to report), but I think one that's easily solved.
> >
> > I've not bisected to precisely which patch (load can take several hours
> > to hit the deadlock), but it doesn't really matter, and I expect that
> > you can guess.
> >
> > My root and home filesystems are ext4 (4kB blocks with 4kB PAGE_SIZE),
> > and so is the filesystem I'm testing, ext4 on /dev/loop0 on tmpfs.
> > So, plenty of ext4 page cache and buffer_heads.
> >
> > Again and again, the deadlock is seen with buffer_migrate_folio_norefs(),
> > either in kcompactd0 or in khugepaged trying to compact, or in both:
> > it ends up calling __lock_buffer(), and that schedules away, waiting
> > forever to get BH_lock.  I have not identified who is holding BH_lock,
> > but I imagine a jbd2 journalling thread, and presume that it wants one
> > of the folio locks which migrate_pages_batch() is already holding; or
> > maybe it's all more convoluted than that.  Other tasks then back up
> > waiting on those folio locks held in the batch.
> >
> > Never a problem with buffer_migrate_folio(), always with the "more
> > careful" buffer_migrate_folio_norefs().  And the patch below fixes
> > it for me: I've had enough hours with it now, on enough occasions,
> > to be confident of that.
> >
> > Cc'ing Jan Kara, who knows buffer_migrate_folio_norefs() and jbd2
> > very well, and I hope can assure us that there is an understandable
> > deadlock here, from holding several random folio locks, then trying
> > to lock buffers.  Cc'ing fsdevel, because there's a risk that mm
> > folk think something is safe, when it's not sufficient to cope with
> > the diversity of filesystems.  I hope nothing more than the below is
> > needed (and I've had no other problems with the patchset: good job),
> > but cannot be sure.
> >
> > [PATCH next] migrate_pages: fix deadlock on buffer heads
> >
> > When __buffer_migrate_folio() is called from buffer_migrate_folio_norefs(),
> > force MIGRATE_ASYNC mode so that buffer_migrate_lock_buffers() will only
> > trylock_buffer(), failing with -EAGAIN as usual if that does not succeed.
> >
> > Signed-off-by: Hugh Dickins <hughd@...gle.com>
> >
> > --- next-20230217/mm/migrate.c
> > +++ fixed/mm/migrate.c
> > @@ -748,7 +748,8 @@ static int __buffer_migrate_folio(struct
> >  	if (folio_ref_count(src) != expected_count)
> >  		return -EAGAIN;
> >  
> > -	if (!buffer_migrate_lock_buffers(head, mode))
> > +	if (!buffer_migrate_lock_buffers(head,
> > +			check_refs ? MIGRATE_ASYNC : mode))
> >  		return -EAGAIN;
> >  
> >  	if (check_refs) {
> 
> Thank you very much for pointing this out and the fix patch.  Today, my
> colleague Pengfei reported a deadlock bug to me.  It seems that we
> cannot wait the writeback to complete when we have locked some folios.
> Below patch can fix that deadlock.  I don't know whether this is related
> to the deadlock you run into.  It appears that we should avoid to
> lock/wait synchronously if we have locked more than one folios.

Thanks, I've checked now, on next-20230217 without my patch but
with your patch below: it took a few hours, but still deadlocks
as I described above, so it's not the same issue.

Yes, that's a good principle, that we should avoid to lock/wait
synchronously once we have locked one folio (hmm, above you say
"more than one": I think we mean the same thing, we're just
stating it differently, given how the code runs at present).

I'm not a great fan of migrate_folio_unmap()'s arguments,
"force" followed by "oh, but don't force" (but applaud the recent
"avoid_force_lock" as much better than the original "force_lock").
I haven't tried, but I wonder if you can avoid both those arguments,
and both of these patches, by passing down an adjusted mode (perhaps
MIGRATE_ASYNC, or perhaps a new mode) to all callees, once the first
folio of a batch has been acquired (then restore to the original mode
when starting a new batch).

(My patch is weak in that it trylocks for buffer_head even on the
first folio of a MIGRATE_SYNC norefs batch, although that has never
given a problem historically: adjusting the mode after acquiring
the first folio would correct that weakness.)

Hugh

> 
> Best Regards,
> Huang, Ying
> 
> ------------------------------------8<------------------------------------
> From 0699fa2f80a67e863107d49a25909c92b900a9be Mon Sep 17 00:00:00 2001
> From: Huang Ying <ying.huang@...el.com>
> Date: Mon, 20 Feb 2023 14:56:34 +0800
> Subject: [PATCH] migrate_pages: fix deadlock on waiting writeback
> 
> Pengfei reported a system soft lockup issue with Syzkaller.  The stack
> traces are as follows,
> 
> ...
> [  300.124933] INFO: task kworker/u4:3:73 blocked for more than 147 seconds.
> [  300.125214]       Not tainted 6.2.0-rc4-kvm+ #1314
> [  300.125408] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  300.125736] task:kworker/u4:3    state:D stack:0     pid:73    ppid:2      flags:0x00004000
> [  300.126059] Workqueue: writeback wb_workfn (flush-7:3)
> [  300.126282] Call Trace:
> [  300.126378]  <TASK>
> [  300.126464]  __schedule+0x43b/0xd00
> [  300.126601]  ? __blk_flush_plug+0x142/0x180
> [  300.126765]  schedule+0x6a/0xf0
> [  300.126912]  io_schedule+0x4a/0x80
> [  300.127051]  folio_wait_bit_common+0x1b5/0x4e0
> [  300.127227]  ? __pfx_wake_page_function+0x10/0x10
> [  300.127403]  __folio_lock+0x27/0x40
> [  300.127541]  write_cache_pages+0x350/0x870
> [  300.127699]  ? __pfx_iomap_do_writepage+0x10/0x10
> [  300.127889]  iomap_writepages+0x3f/0x80
> [  300.128037]  xfs_vm_writepages+0x94/0xd0
> [  300.128192]  ? __pfx_xfs_vm_writepages+0x10/0x10
> [  300.128370]  do_writepages+0x10a/0x240
> [  300.128514]  ? lock_is_held_type+0xe6/0x140
> [  300.128675]  __writeback_single_inode+0x9f/0xa90
> [  300.128854]  writeback_sb_inodes+0x2fb/0x8d0
> [  300.129030]  __writeback_inodes_wb+0x68/0x150
> [  300.129212]  wb_writeback+0x49c/0x770
> [  300.129357]  wb_workfn+0x6fb/0x9d0
> [  300.129500]  process_one_work+0x3cc/0x8d0
> [  300.129669]  worker_thread+0x66/0x630
> [  300.129824]  ? __pfx_worker_thread+0x10/0x10
> [  300.129989]  kthread+0x153/0x190
> [  300.130116]  ? __pfx_kthread+0x10/0x10
> [  300.130264]  ret_from_fork+0x29/0x50
> [  300.130409]  </TASK>
> [  300.179347] INFO: task repro:1023 blocked for more than 147 seconds.
> [  300.179905]       Not tainted 6.2.0-rc4-kvm+ #1314
> [  300.180317] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  300.180955] task:repro           state:D stack:0     pid:1023  ppid:360    flags:0x00004004
> [  300.181660] Call Trace:
> [  300.181879]  <TASK>
> [  300.182085]  __schedule+0x43b/0xd00
> [  300.182407]  schedule+0x6a/0xf0
> [  300.182694]  io_schedule+0x4a/0x80
> [  300.183020]  folio_wait_bit_common+0x1b5/0x4e0
> [  300.183506]  ? compaction_alloc+0x77/0x1150
> [  300.183892]  ? __pfx_wake_page_function+0x10/0x10
> [  300.184304]  folio_wait_bit+0x30/0x40
> [  300.184640]  folio_wait_writeback+0x2e/0x1e0
> [  300.185034]  migrate_pages_batch+0x555/0x1ac0
> [  300.185462]  ? __pfx_compaction_alloc+0x10/0x10
> [  300.185808]  ? __pfx_compaction_free+0x10/0x10
> [  300.186022]  ? __this_cpu_preempt_check+0x17/0x20
> [  300.186234]  ? lock_is_held_type+0xe6/0x140
> [  300.186423]  migrate_pages+0x100e/0x1180
> [  300.186603]  ? __pfx_compaction_free+0x10/0x10
> [  300.186800]  ? __pfx_compaction_alloc+0x10/0x10
> [  300.187011]  compact_zone+0xe10/0x1b50
> [  300.187182]  ? lock_is_held_type+0xe6/0x140
> [  300.187374]  ? check_preemption_disabled+0x80/0xf0
> [  300.187588]  compact_node+0xa3/0x100
> [  300.187755]  ? __sanitizer_cov_trace_const_cmp8+0x1c/0x30
> [  300.187993]  ? _find_first_bit+0x7b/0x90
> [  300.188171]  sysctl_compaction_handler+0x5d/0xb0
> [  300.188376]  proc_sys_call_handler+0x29d/0x420
> [  300.188583]  proc_sys_write+0x2b/0x40
> [  300.188749]  vfs_write+0x3a3/0x780
> [  300.188912]  ksys_write+0xb7/0x180
> [  300.189070]  __x64_sys_write+0x26/0x30
> [  300.189260]  do_syscall_64+0x3b/0x90
> [  300.189424]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> [  300.189654] RIP: 0033:0x7f3a2471f59d
> [  300.189815] RSP: 002b:00007ffe567f7288 EFLAGS: 00000217 ORIG_RAX: 0000000000000001
> [  300.190137] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3a2471f59d
> [  300.190397] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
> [  300.190653] RBP: 00007ffe567f72a0 R08: 0000000000000010 R09: 0000000000000010
> [  300.190910] R10: 0000000000000010 R11: 0000000000000217 R12: 00000000004012e0
> [  300.191172] R13: 00007ffe567f73e0 R14: 0000000000000000 R15: 0000000000000000
> [  300.191440]  </TASK>
> ...
> 
> To migrate a folio, we may wait the writeback of a folio to complete
> when we already have held the lock of some folios.  But the writeback
> code may wait to lock some folio we held lock.  This causes the
> deadlock.  To fix the issue, we will avoid to wait the writeback to
> complete if we have locked some folios.  After moving the locked
> folios and unlocked, we will retry.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Reported-by: "Xu, Pengfei" <pengfei.xu@...el.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Stefan Roesch <shr@...kernel.io>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Xin Hao <xhao@...ux.alibaba.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Yang Shi <shy828301@...il.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  mm/migrate.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 28b435cdeac8..bc9a8050f1b0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1205,6 +1205,18 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>  		}
>  		if (!force)
>  			goto out;
> +		/*
> +		 * We have locked some folios and are going to wait the
> +		 * writeback of this folio to complete.  But it's possible for
> +		 * the writeback to wait to lock the folios we have locked.  To
> +		 * avoid a potential deadlock, let's bail out and not do that.
> +		 * The locked folios will be moved and unlocked, then we
> +		 * can wait the writeback of this folio.
> +		 */
> +		if (avoid_force_lock) {
> +			rc = -EDEADLOCK;
> +			goto out;
> +		}
>  		folio_wait_writeback(src);
>  	}
>  
> -- 
> 2.39.1
> 
> 

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