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Message-Id: <C5CED45A-FFDA-46C7-A116-284431CF5940@lca.pw>
Date: Sun, 8 Mar 2020 23:27:22 -0400
From: Qian Cai <cai@....pw>
To: Rafael Aquini <aquini@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Linux Memory Management List <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
mgorman <mgorman@...hsingularity.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...e.com>,
Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm, numa: fix bad pmd by atomically check for
pmd_trans_huge when marking page tables prot_numa
> On Mar 8, 2020, at 7:14 PM, Rafael Aquini <aquini@...hat.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@...hat.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@....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@....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))
return 0;
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@...hsingularity.net>
>>>>> Cc: stable@...r.kernel.org
>>>>> Signed-off-by: Rafael Aquini <aquini@...hat.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
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