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Date:   Thu, 21 May 2020 12:48:58 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Qian Cai <cai@lca.pw>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Zi Yan <ziy@nvidia.com>, John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH mmotm] mm/vmstat: Add events for PMD based THP migration
 without split fix
In-Reply-To: <20200521185419.GB6367@ovpn-112-192.phx2.redhat.com>
Message-ID: <alpine.LSU.2.11.2005211232170.1158@eggly.anvils>
References: <alpine.LSU.2.11.2005210643340.482@eggly.anvils> <20200521185419.GB6367@ovpn-112-192.phx2.redhat.com>
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On Thu, 21 May 2020, Qian Cai wrote:
> On Thu, May 21, 2020 at 06:49:51AM -0700, Hugh Dickins wrote:
> > Fix 5.7-rc6-mm1 page migration crash in unmap_and_move(): when the
> > page to be migrated has been freed from under us, that is considered 
> > a MIGRATEPAGE_SUCCESS, but no newpage has been allocated (and I don't
> > think it would ever need to be counted as a successful THP migration).
> > 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> > Fix to mm-vmstat-add-events-for-pmd-based-thp-migration-without-split.patch
> > 
> >  mm/migrate.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- 5.7-rc6-mm1/mm/migrate.c	2020-05-20 12:21:56.117693827 -0700
> > +++ linux/mm/migrate.c	2020-05-20 15:08:12.319476978 -0700
> > @@ -1248,7 +1248,7 @@ out:
> >  	 * we want to retry.
> >  	 */
> >  	if (rc == MIGRATEPAGE_SUCCESS) {
> > -		if (PageTransHuge(newpage))
> > +		if (newpage && PageTransHuge(newpage))
> 
> Should this be
> 
> if (!IS_ERR_OR_NULL(newpage) && PageTransHuge(newpage)) ?

No: newpage is set higher up by:

	newpage = get_new_page(page, private);
	if (!newpage)
		return -ENOMEM;

and there's enough other places after that which would crash on an
erroneous newpage.  (Of course, the get_new_page() convention might
be changed in future to return an error code, but no need to make
such a change here and now.)

> 
> I have also crashed here due to the buggy commit,
> 
> unmap_and_move() -> PageTransHuge(page) -> page->compound_head
> 
> but it said 0x00000008 instead of NULL which is aweful a lot like,

0x00000008 is where it's trying to dereference NULL->compound_head,
no mystery there.

> 
> https://lore.kernel.org/linux-mm/20200512215813.GA487759@cmpxchg.org/
> 
> Interesting thing is I applied this patch and the problem went away, but not
> sure if it could still be ERR_PTR sometimes just not always?
> 
> [  210.929981][ T4159] BUG: Kernel NULL pointer dereference on read at 0x00000008
> [  210.930009][ T4159] Faulting instruction address: 0xc0000000005196c8
> [  210.930027][   C61] irq event stamp: 270727
> [  210.930028][ T4159] Oops: Kernel access of bad area, sig: 11 [#1]
> [  210.930033][ T4159] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=256 DEBUG_PAGEALLOC NUMA PowerNV
> [  210.930058][   C61] hardirqs last  enabled at (270726): [<c000000000adcc44>] _raw_spin_unlock_irqrestore+0x94/0xd0
> [  210.930064][ T4159] Modules linked in: kvm_hv kvm ip_tables x_tables xfs sd_mod tg3 bnx2x libphy mdio firmware_class ahci libahci libata dm_mirror dm_region_hash dm_log dm_mod
> [  210.930083][   C61] hardirqs last disabled at (270727): [<c000000000adc8dc>] _raw_spin_lock_irqsave+0x3c/0xa0
> [  210.930086][   C61] softirqs last  enabled at (270556): [<c000000000addbdc>] __do_softirq+0x6dc/0xaa8
> [  210.930125][ T4159] CPU: 109 PID: 4159 Comm: test.sh Not tainted 5.7.0-rc6-next-20200521+ #112
> [  210.930163][   C61] softirqs last disabled at (270559): [<c000000000129544>] run_ksoftirqd+0x74/0xc0
> [  210.930260][ T4159] NIP:  c0000000005196c8 LR: c000000000519568 CTR: 0000000000000000
> [  210.930307][ T4159] REGS: c0002005b10af570 TRAP: 0300   Not tainted  (5.7.0-rc6-next-20200521+)
> [  210.930342][ T4159] MSR:  900000000280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 24248242  XER: 00000000
> [  210.930403][ T4159] CFAR: c000000000519570 DAR: 0000000000000008 DSISR: 40000000 IRQMASK: 0 
> [  210.930403][ T4159] GPR00: c000000000519568 c0002005b10af800 c000000001765500 0000000000000000 
> [  210.930403][ T4159] GPR04: c000000001c18d28 0000000000000006 0000000035279396 fffffffef69809d2 
> [  210.930403][ T4159] GPR08: 0000201cc6240000 0000000000000000 0000000000000000 0000000000000008 
> [  210.930403][ T4159] GPR12: 0000000000008000 c000201fff670600 0000000000000000 c00c000805312a00 
> [  210.930403][ T4159] GPR16: c00000000050b090 c000000000c857d0 c00c000805312a80 0000000000000000 
> [  210.930403][ T4159] GPR20: 0000000000000001 0000000000000000 c0002005b10af978 c000201ffc7c8780 
> [  210.930403][ T4159] GPR24: fffffffffffffff5 0000000000000002 0000000020000000 0000000000000000 
> [  210.930403][ T4159] GPR28: 0000000000000007 0000000000000001 0000000000000000 c00c000805312a08 
> [  210.930740][ T4159] NIP [c0000000005196c8] migrate_pages+0xc18/0x1ad0
> [  210.930775][ T4159] LR [c000000000519568] migrate_pages+0xab8/0x1ad0
> [  210.930824][ T4159] Call Trace:
> [  210.930843][ T4159] [c0002005b10af800] [c000000000519568] migrate_pages+0xab8/0x1ad0 (unreliable)
> [  210.930882][ T4159] [c0002005b10af910] [c00000000050b6fc] do_migrate_range+0x25c/0x8f0
> [  210.930940][ T4159] [c0002005b10afa10] [c00000000050e974] __offline_pages+0x6e4/0x8b0
> [  210.930988][ T4159] [c0002005b10afb40] [c000000000887f6c] memory_block_action+0xac/0xc0
> [  210.931016][ T4159] [c0002005b10afba0] [c000000000888618] memory_subsys_offline+0x58/0xa0
> [  210.931030][ T4159] [c0002005b10afbd0] [c0000000008621a0] device_offline+0x100/0x140
> [  210.931080][ T4159] [c0002005b10afc10] [c000000000888938] state_store+0x108/0x190
> [  210.931128][ T4159] [c0002005b10afc50] [c00000000085b628] dev_attr_store+0x38/0x60
> [  210.931176][ T4159] [c0002005b10afc70] [c0000000006b9790] sysfs_kf_write+0x70/0xb0
> [  210.931211][ T4159] [c0002005b10afcb0] [c0000000006b895c] kernfs_fop_write+0x11c/0x270
> [  210.931249][ T4159] [c0002005b10afd00] [c00000000057bcac] __vfs_write+0x3c/0x70
> [  210.931273][ T4159] [c0002005b10afd20] [c00000000057f0ac] vfs_write+0xcc/0x200
> [  210.931319][ T4159] [c0002005b10afd70] [c00000000057f44c] ksys_write+0x7c/0x140
> [  210.931345][ T4159] [c0002005b10afdc0] [c000000000039e78] system_call_exception+0x108/0x1d0
> [  210.931395][ T4159] [c0002005b10afe20] [c00000000000c9f0] system_call_common+0xf0/0x278
> [  210.931445][ T4159] Instruction dump:
> [  210.931476][ T4159] 9bad0988 e90d0028 3d22ff9f 3929c670 7d49402a 394a0001 7d49412a 4bafdec5 
> [  210.931492][ T4159] 60000000 4bfff544 2fbe0000 409e04c4 <e93b0008> 71290001 40820928 e93b0000 
> [  210.931521][ T4159] ---[ end trace 03092b3800dbb5cb ]---
> [  211.416724][ T4159] 
> [  212.416810][ T4159] Kernel panic - not syncing: Fatal exception
> [  213.829268][ T4159] ---[ end Kernel panic - not syncing: Fatal exception ]---
> 
> >  			thp_migration_success(true);
> >  		put_page(page);
> >  		if (reason == MR_MEMORY_FAILURE) {
> >