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Date:   Mon, 9 Mar 2020 11:05:31 -0400
From:   Rafael Aquini <aquini@redhat.com>
To:     Qian Cai <cai@lca.pw>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        mgorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm, numa: fix bad pmd by atomically check for
 pmd_trans_huge when marking page tables prot_numa
Message-ID: <20200309150531.GD25642@optiplex-lnx>
References: <20200216191800.22423-1-aquini@redhat.com>
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 <20200307030530.GB4093@t490s>
 <9124C8B9-FB47-44F5-8606-DD9261BCB383@lca.pw>
 <20200308231423.GA22348@t490s>
 <C5CED45A-FFDA-46C7-A116-284431CF5940@lca.pw>
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On Sun, Mar 08, 2020 at 11:27:22PM -0400, Qian Cai wrote:
> 
> 
> > On Mar 8, 2020, at 7:14 PM, Rafael Aquini <aquini@redhat.com> wrote:
> > 
> > On Sat, Mar 07, 2020 at 10:20:20PM -0500, Qian Cai wrote:
> >> Not sure if it is really worth hurrying this into -rc5 now given...
> >> 
> >>> On Mar 6, 2020, at 10:05 PM, Rafael Aquini <aquini@redhat.com> wrote:
> >>>> 
> >>>> After reverted this patch, it is no longer possible to reproduce a bug under memory pressure
> >>>> with swapping on linux-next 20200306,
> >>>> 
> >>> 
> >>> Would you mind sharing your .config ? Also it would be nice to know your
> >>> system and workload details so I can try to reproduce.
> >> 
> >> LTP oom02 in a tight loop,
> >> 
> >> # i=0; while :; echo $((i++)); ./oom02; sleep 10; done
> >> 
> >> HPE ProLiant DL385 Gen10
> >> AMD EPYC 7601 32-Core Processor
> >> 65536 MB memory, 400 GB disk space
> >> 
> >> Processors	128
> >> Cores	64
> >> Sockets	2
> >> 
> >> linux-next 20200306
> >> 
> > 
> > Thx! what about your .config?
> 
> https://raw.githubusercontent.com/cailca/linux-mm/master/x86.config
> 
> My theory is this race,
> 
> From 73d4e8dad6cb248970bade5049cc072cd9090d0f Mon Sep 17 00:00:00 2001
> From: Qian Cai <cai@lca.pw>
> Date: Sun, 8 Mar 2020 17:37:51 -0400
> Subject: [PATCH] mm/mprotect: fix a race in change_pmd_range()
> 
> The recent commit increased the race window between,
> 
> CPU0:
> task_numa_work()
> change_prot_numa()
> change_protection()
> change_protection_range()
> change_pmd_range()
> pmd_none_or_clear_bad_unless_trans_huge(pmd) <-- PMD is NULL.
> 		goto next;
> 				CPU1:
> 				do_huge_pmd_anonymous_page()
> 				__do_huge_pmd_anonymous_page()
> 				PMD is transhuge.
> 
> where it will not calling __split_huge_pmd() properly and trigger a
> crash later on,
> 
> kernel BUG at include/linux/page-flags.h:317!
> 
> end_swap_bio_write()
>   SetPageError(page)
>       VM_BUG_ON_PAGE(1 && PageCompound(page))
> 
> Fix it by using a similar check in pmd_trans_unstable().
> 
> Fixes: 8b272b3cbbb5 ("mm, numa: fix bad pmd by atomically check for pmd_trans_huge when marking page tables prot_numa")
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  mm/mprotect.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 311c0dadf71c..00dd2c66a434 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -165,7 +165,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>   * Used when setting automatic NUMA hinting protection where it is
>   * critical that a numa hinting PMD is not confused with a bad PMD.
>   */
> -static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd)
> +static inline int pmd_clear_bad_unless_none_or_trans_huge(pmd_t *pmd)
>  {
>  	pmd_t pmdval = pmd_read_atomic(pmd);
>  
> @@ -174,9 +174,7 @@ static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd)
>  	barrier();
>  #endif
>  
> -	if (pmd_none(pmdval))
> -		return 1;
> -	if (pmd_trans_huge(pmdval))
> +	if (pmd_none(pmdval) || pmd_trans_huge(pmdval))


I'm still waiting on a similar system to become available, so I can
work on your reproducer case, as well as to dig and wrap my head around it.

I still don't think that skipping the pmd_none() in the change-protection walk 
should cause a big fuss like you observed here (yet, it seems it does), and 
the fact that we need that race window to take the __split_huge_pmd() suggests,
at least to me, that we might be missing this proper split somewhere else.

>  	if (unlikely(pmd_bad(pmdval))) {
>  		pmd_clear_bad(pmd);
> @@ -213,7 +211,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>  		 * for all the checks.
>  		 */
>  		if (!is_swap_pmd(*pmd) && !pmd_devmap(*pmd) &&
> -		     pmd_none_or_clear_bad_unless_trans_huge(pmd))
> +		     pmd_clear_bad_unless_none_or_trans_huge(pmd))
>  			goto next;
>  
>  		/* invoke the mmu notifier if the pmd is populated */
> -- 
> 2.21.0 (Apple Git-122.2)
> 
> 
> > 
> > 
> > 
> > Cheers,
> > -- Rafael
> > 
> >>>> The trace is a bit off for some reasons, but it is basically,
> >>>> 
> >>>> kernel BUG at include/linux/page-flags.h:317!
> >>>> 
> >>>> end_swap_bio_write()
> >>>>  SetPageError(page)
> >>>>      VM_BUG_ON_PAGE(1 && PageCompound(page)) 
> >>>> 
> >>>> The page was allocated in,
> >>>> 
> >>>> do_huge_pmd_anonymous_page()
> >>>> 
> >>>> [ 9598.551238][  T794] oom_reaper: reaped process 49120 (oom02), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB 
> >>>> [ 9690.896638][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc0000 flags=0x0010] 
> >>>> [ 9690.910129][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc1000 flags=0x0010] 
> >>>> [ 9690.923933][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc1900 flags=0x0010] 
> >>>> [ 9690.937669][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc1d00 flags=0x0010] 
> >>>> [ 9690.951468][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc2000 flags=0x0010] 
> >>>> [ 9690.964803][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc2400 flags=0x0010] 
> >>>> [ 9690.978256][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc2800 flags=0x0010] 
> >>>> [ 9690.991423][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc2c00 flags=0x0010] 
> >>>> [ 9691.004294][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc3000 flags=0x0010] 
> >>>> [ 9691.017228][  T814] smartpqi 0000:23:00.0: AMD-Vi: Event logged [IO_PAGE_FAULT domain=0x0027 address=0xffffffffffcc3400 flags=0x0010] 
> >>>> [ 9691.029804][  T814] AMD-Vi: Event logged [IO_PAGE_FAULT device=23:00.0 domain=0x0027 address=0xffffffffffcc3800 flags=0x0010] 
> >>>> [ 9691.041623][  T814] AMD-Vi: Event logged [IO_PAGE_FAULT device=23:00.0 domain=0x0027 address=0xffffffffffcc3c00 flags=0x0010] 
> >>>> [ 9692.767064][    C8] smartpqi 0000:23:00.0: controller is offline: status code 0x14803 
> >>>> [ 9692.775562][    C8] smartpqi 0000:23:00.0: controller offline 
> >>>> [ 9692.841721][   C36] sd 0:1:0:0: [sda] tag#693 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.841970][   C87] blk_update_request: I/O error, dev sda, sector 59046400 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.842718][   C31] blk_update_request: I/O error, dev sda, sector 36509696 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.842748][   C18] sd 0:1:0:0: [sda] tag#616 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.842757][   C28] sd 0:1:0:0: [sda] tag#841 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.842766][  C109] sd 0:1:0:0: [sda] tag#393 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.842811][  C109] sd 0:1:0:0: [sda] tag#393 CDB: opcode=0x2a 2a 00 01 60 c8 00 00 02 00 00 
> >>>> [ 9692.842840][   C13] sd 0:1:0:0: [sda] tag#324 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.842924][   C18] sd 0:1:0:0: [sda] tag#616 CDB: opcode=0x2a 2a 00 01 c7 8c 00 00 02 00 00 
> >>>> [ 9692.842932][  C109] blk_update_request: I/O error, dev sda, sector 23119872 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.842938][   C28] sd 0:1:0:0: [sda] tag#841 CDB: opcode=0x2a 2a 00 03 2f cc 00 00 02 00 00 
> >>>> [ 9692.842984][  T213] sd 0:1:0:0: [sda] tag#47 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.842995][   C28] blk_update_request: I/O error, dev sda, sector 53464064 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843000][   C81] sd 0:1:0:0: [sda] tag#10 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.843031][   C18] blk_update_request: I/O error, dev sda, sector 29854720 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843056][   C13] sd 0:1:0:0: [sda] tag#324 CDB: opcode=0x2a 2a 00 03 17 66 00 00 02 00 00 
> >>>> [ 9692.843101][   C81] sd 0:1:0:0: [sda] tag#10 CDB: opcode=0x2a 2a 00 00 52 a4 00 00 02 00 00 
> >>>> [ 9692.843129][   C13] blk_update_request: I/O error, dev sda, sector 51865088 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843192][   C81] blk_update_request: I/O error, dev sda, sector 5415936 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843338][  T213] sd 0:1:0:0: [sda] tag#47 CDB: opcode=0x2a 2a 00 02 be 5a 00 00 02 00 00 
> >>>> [ 9692.843358][   C13] sd 0:1:0:0: [sda] tag#325 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.843480][  T213] blk_update_request: I/O error, dev sda, sector 46029312 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843485][   C13] sd 0:1:0:0: [sda] tag#325 CDB: opcode=0x2a 2a 00 03 17 68 00 00 02 00 00 
> >>>> [ 9692.843510][   C13] blk_update_request: I/O error, dev sda, sector 51865600 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843685][  C103] blk_update_request: I/O error, dev sda, sector 34941952 op 0x1:(WRITE) flags 0x8004000 phys_seg 4 prio class 0 
> >>>> [ 9692.843725][   C96] sd 0:1:0:0: [sda] tag#877 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.843744][   C92] Write-error on swap-device (254:1:39256632) 
> >>>> [ 9692.843791][  C118] sd 0:1:0:0: [sda] tag#230 UNKNOWN(0x2003) Result: hostbyte=0x01 driverbyte=0x00 cmd_age=1s 
> >>>> [ 9692.843866][   C64] Write-error on swap-device (254:1:63030016) 
> >>>> [ 9692.843891][   C96] sd 0:1:0:0: [sda] tag#877 CDB: opcode=0x2a 2a 00 03 1e 48 00 00 02 00 00 
> >>>> [ 9692.844013][  C118] sd 0:1:0:0: [sda] tag#230 CDB: opcode=0x2a 2a 00 03 c6 26 00 00 02 00 00 
> >>>> [ 9692.844615][   C92] Write-error on swap-device (254:1:39256640) 
> >>>> [ 9692.844643][   C64] Write-error on swap-device (254:1:63030024) 
> >>>> [ 9692.845109][   C61] Write-error on swap-device (254:1:17105384) 
> >>>> [ 9692.845192][   C25] page:fffff3b2ec3a8000 refcount:512 mapcount:0 mapping:000000009eb0338c index:0x7f6e58200 head:fffff3b2ec3a8000 order:9 compound_mapcount:0 compound_pincount:0 
> >>>> [ 9692.845226][   C25] anon flags: 0x45fffe0000d8454(uptodate|lru|workingset|owner_priv_1|writeback|head|reclaim|swapbacked) 
> >>>> [ 9692.845254][   C25] raw: 045fffe0000d8454 fffff3b2f9d48008 fffff3b2f3270008 ffff97c3090f8209 
> >>>> [ 9692.845283][   C25] raw: 00000007f6e58200 000000000053c800 00000200ffffffff ffff97c8d71e9000 
> >>>> [ 9692.845293][  C122] Write-error on swap-device (254:1:39080184) 
> >>>> [ 9692.845316][  T213] page:fffff3b2f77d8000 refcount:512 mapcount:0 mapping:00000000a4517f4e index:0x7f69d2200 head:fffff3b2f77d8000 order:9 compound_mapcount:0 compound_pincount:0 
> >>>> [ 9692.845330][   C25] page dumped because: VM_BUG_ON_PAGE(1 && PageCompound(page)) 
> >>>> [ 9692.845427][  T213] anon flags: 0x45fffe0000d8454(uptodate|lru|workingset|owner_priv_1|writeback|head|reclaim|swapbacked) 
> >>>> [ 9692.845433][   C25] page->mem_cgroup:ffff97c8d71e9000 
> >>>> [ 9692.845443][   C25] page_owner tracks the page as allocated 
> >>>> [ 9692.845520][  T213] raw: 045fffe0000d8454 fffff3b2f5ec2f08 fffff3b2f6b88008 ffff97c715cc0609 
> >>>> [ 9692.845535][   C25] page last allocated via order 9, migratetype Movable, gfp_mask 0x1c20ca(GFP_TRANSHUGE_LIGHT) 
> >>>> [ 9692.845606][   C25]  prep_new_page+0x1cd/0x1f0 
> >>>> [ 9692.845628][   C25]  get_page_from_freelist+0x18ac/0x24d0 
> >>>> [ 9692.845639][  T213] raw: 00000007f69d2200 00000000000bf400 00000200ffffffff ffff97c8d71e9000 
> >>>> [ 9692.845654][   C25]  __alloc_pages_nodemask+0x1b1/0x450 
> >>>> [ 9692.845733][  T213] page dumped because: VM_BUG_ON_PAGE(1 && PageCompound(page)) 
> >>>> [ 9692.845757][   C25]  alloc_pages_vma+0x8a/0x2c0 
> >>>> [ 9692.845762][  T213] page->mem_cgroup:ffff97c8d71e9000 
> >>>> [ 9692.845779][   C25]  do_huge_pmd_anonymous_page+0x1d4/0xc30 
> >>>> [ 9692.845786][  T213] page_owner tracks the page as allocated 
> >>>> [ 9692.845807][   C25]  __handle_mm_fault+0xd27/0xd50 
> >>>> [ 9692.845819][  T213] page last allocated via order 9, migratetype Movable, gfp_mask 0x1c20ca(GFP_TRANSHUGE_LIGHT) 
> >>>> [ 9692.845833][   C25]  handle_mm_fault+0xfc/0x2f0 
> >>>> [ 9692.845856][  T213]  prep_new_page+0x1cd/0x1f0 
> >>>> [ 9692.845876][   C25]  do_page_fault+0x263/0x6f9 
> >>>> [ 9692.845892][  T213]  get_page_from_free7219][   C25]  clone_endio+0xe4/0x2c0 [dm_mod] 
> >>>> [ 9692.847226][  T213] kernel BUG at include/linux/page-flags.h:317! 
> >>>> [ 9692.847234][   C64] Write-error on swap-device (254:1:63030064) 
> >>>> [ 9692.847250][   C25]  bio_endio+0x297/0x560 
> >>>> [ 9692.847255][   C61] Write-error on swap-device (254:1:17105424) 
> >>>> [ 9692.847263][   C25]  ? bio_advance+0x92/0x190 
> >>>> [ 9692.847267][  C122] Write-error on swap-device (254:1:39080224) 
> >>>> [ 9692.847276][   C25]  blk_update_request+0x201/0x920 
> >>>> [ 9692.847287][   C25]  ? ___ratelimit+0x138/0x1e0 
> >>>> [ 9692.847312][   C25]  scsi_end_request+0x6b/0x4b0 
> >>>> [ 9692.847320][   C25]  ? ___ratelimit+0x3e/0x1e0 
> >>>> [ 9692.847329][   C25]  scsi_io_completion+0x509/0x7e0 
> >>>> [ 9692.847340][   C25]  scsi_finish_command+0x1ed/0x2a0 
> >>>> [ 9692.847350][   C25]  scsi_softirq_done+0x1c9/0x1d0 
> >>>> [ 9692.847354][   C64] Write-error on swap-device (254:1:63030072) 
> >>>> [ 9692.847363][   C25]  ? blk_mq_check_inflight+0xa0/0xa0 
> >>>> [ 9692.847383][   C61] Write-error on swap-device (254:1:17105432) 
> >>>> [ 9692.847397][  C122] Write-erro692.848242][   C25] RSP: 0018:ffffb02006b68bb0 EFLAGS: 00010086 
> >>>> [ 9692.848333][  T213] Workqueue: events pqi_ctrl_offline_worker [smartpqi] 
> >>>> [ 9692.848346][   C61] Write-error on swap-device (254:1:17105528) 
> >>>> [ 9692.848351][  C122] Write-error on swap-device (254:1:39080328) 
> >>>> [ 9692.848428][   C25] RAX: 0000000000000000 RBX: ffff97c76d8d4b80 RCX: ffffffff9d51e8f0 
> >>>> [ 9692.848433][   C61] Write-error on swap-device (254:1:17105536) 
> >>>> [ 9692.848437][  C122] Write-error on swap-device (254:1:39080336) 
> >>>> [ 9692.848445][  T213] RIP: 0010:end_swap_bio_write+0x90/0x280 
> >>>> [ 9692.848450][   C25] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff97c7e539f00a 
> >>>> [ 9692.848461][  T213] Code: 80 7b 1a 00 0f 84 58 01 00 00 4c 89 e6 bf 10 00 00 00 e8 33 f8 ff ff 84 c0 74 1d 48 c7 c6 18 7e 3b 9a 4c 89 e7 e8 20 78 fa ff <0f> 0b 48 c7 c7 b0 0b 6e 9a e8 04 ee 2b 00 4d 8d 74 24 08 ba 08 00 
> >>>> [ 9692.848466][   C25] RBP: ffffb02006b68bf0 R08: 0000000000000000 R09: 000097c7e539f00a 
> >>>> [ 9692.848471][  T213] RSP: 0018:ffffb02006dd3908 EFLAGS: 00010286 
> >>>> [ 9692.848485][   C25] R10: 0000b02006b689a0 R11: 000097c7e5ax_pmem_core efivars acpi_cpufreq efivarfs ip_tables x_tables xfs sd_mod smartpqi scsi_transport_sas tg3 mlx5_core 
> >>>> [ 9692.849818][  C122] Write-error on swap-device (254:1:39080440) 
> >>>> [ 9692.849884][  T213]  libphy 
> >>>> [ 9692.849889][  C122] Write-error on swap-device (254:1:39080448) 
> >>>> [ 9692.849952][  T213]  firmware_class 
> >>>> [ 9692.849959][  C122] Write-error on swap-device (254:1:39080456) 
> >>>> [ 9692.849963][  T213]  dm_mirror dm_region_hash dm_log dm_mod 
> >>>> [ 9692.849990][  C122] Write-error on swap-device (254:1:39080464) 
> >>>> [ 9692.850069][  C122] Write-error on swap-device (254:1:39080472) 
> >>>> [ 9692.850107][  C122] Write-error on swap-device (254:1:39080480) 
> >>>> [ 9692.850156][  C122] Write-error on swap-device (254:1:39080488) 
> >>>> [ 9692.850220][  C122] Write-error on swap-device (254:1:39080496) 
> >>>> [ 9692.850264][  C122] Write-error on swap-device (254:1:39080504) 
> >>>> [ 9692.850323][  C122] Write-error on swap-device (254:1:39080512) 
> >>>> [ 9692.850361][  C122] Write-error on swap-device (254:1:39080520) 
> >>>> [ 9692.850411][  C122] Write-error on swap-device (254:1:39080528)  
> >>>> 
> >>>>> 
> >>>>> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> >>>>> Cc: stable@vger.kernel.org
> >>>>> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> >>>>> ---
> >>>>> mm/mprotect.c | 38 ++++++++++++++++++++++++++++++++++++--
> >>>>> 1 file changed, 36 insertions(+), 2 deletions(-)
> >>>>> 
> >>>>> diff --git a/mm/mprotect.c b/mm/mprotect.c
> >>>>> index 7a8e84f86831..9ea8cc0ab2fd 100644
> >>>>> --- a/mm/mprotect.c
> >>>>> +++ b/mm/mprotect.c
> >>>>> @@ -161,6 +161,31 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >>>>> 	return pages;
> >>>>> }
> >>>>> 
> >>>>> +/*
> >>>>> + * Used when setting automatic NUMA hinting protection where it is
> >>>>> + * critical that a numa hinting PMD is not confused with a bad PMD.
> >>>>> + */
> >>>>> +static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd)
> >>>>> +{
> >>>>> +	pmd_t pmdval = pmd_read_atomic(pmd);
> >>>>> +
> >>>>> +	/* See pmd_none_or_trans_huge_or_clear_bad for info on barrier */
> >>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >>>>> +	barrier();
> >>>>> +#endif
> >>>>> +
> >>>>> +	if (pmd_none(pmdval))
> >>>>> +		return 1;
> >>>>> +	if (pmd_trans_huge(pmdval))
> >>>>> +		return 0;
> >>>>> +	if (unlikely(pmd_bad(pmdval))) {
> >>>>> +		pmd_clear_bad(pmd);
> >>>>> +		return 1;
> >>>>> +	}
> >>>>> +
> >>>>> +	return 0;
> >>>>> +}
> >>>>> +
> >>>>> static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> >>>>> 		pud_t *pud, unsigned long addr, unsigned long end,
> >>>>> 		pgprot_t newprot, int dirty_accountable, int prot_numa)
> >>>>> @@ -178,8 +203,17 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> >>>>> 		unsigned long this_pages;
> >>>>> 
> >>>>> 		next = pmd_addr_end(addr, end);
> >>>>> -		if (!is_swap_pmd(*pmd) && !pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)
> >>>>> -				&& pmd_none_or_clear_bad(pmd))
> >>>>> +
> >>>>> +		/*
> >>>>> +		 * Automatic NUMA balancing walks the tables with mmap_sem
> >>>>> +		 * held for read. It's possible a parallel update to occur
> >>>>> +		 * between pmd_trans_huge() and a pmd_none_or_clear_bad()
> >>>>> +		 * check leading to a false positive and clearing.
> >>>>> +		 * Hence, it's ecessary to atomically read the PMD value
> >>>>> +		 * for all the checks.
> >>>>> +		 */
> >>>>> +		if (!is_swap_pmd(*pmd) && !pmd_devmap(*pmd) &&
> >>>>> +		     pmd_none_or_clear_bad_unless_trans_huge(pmd))
> >>>>> 			goto next;
> >>>>> 
> >>>>> 		/* invoke the mmu notifier if the pmd is populated */
> >>>>> -- 
> >>>>> 2.24.1
>