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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 6 Aug 2019 14:11:56 -0700
Message-ID: <CAPcyv4jgtYMKgEB4jnQ0g4fQPO39BCOmQM8Zo231=_D7L6wH=A@mail.gmail.com>
Subject: Re: [PATCH v4 2/2] mm/memory-failure: Poison read receives SIGKILL
 instead of SIGBUS if mmaped more than once
To:     Jane Chu <jane.chu@oracle.com>
Cc:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>
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On Tue, Aug 6, 2019 at 10:28 AM Jane Chu <jane.chu@oracle.com> wrote:
>
> Mmap /dev/dax more than once, then read the poison location using address
> from one of the mappings. The other mappings due to not having the page
> mapped in will cause SIGKILLs delivered to the process. SIGKILL succeeds
> over SIGBUS, so user process looses the opportunity to handle the UE.
>
> Although one may add MAP_POPULATE to mmap(2) to work around the issue,
> MAP_POPULATE makes mapping 128GB of pmem several magnitudes slower, so
> isn't always an option.
>
> Details -
>
> ndctl inject-error --block=10 --count=1 namespace6.0
>
> ./read_poison -x dax6.0 -o 5120 -m 2
> mmaped address 0x7f5bb6600000
> mmaped address 0x7f3cf3600000
> doing local read at address 0x7f3cf3601400
> Killed
>
> Console messages in instrumented kernel -
>
> mce: Uncorrected hardware memory error in user-access at edbe201400
> Memory failure: tk->addr = 7f5bb6601000
> Memory failure: address edbe201: call dev_pagemap_mapping_shift
> dev_pagemap_mapping_shift: page edbe201: no PUD
> Memory failure: tk->size_shift == 0
> Memory failure: Unable to find user space address edbe201 in read_poison
> Memory failure: tk->addr = 7f3cf3601000
> Memory failure: address edbe201: call dev_pagemap_mapping_shift
> Memory failure: tk->size_shift = 21
> Memory failure: 0xedbe201: forcibly killing read_poison:22434 because of failure to unmap corrupted page
>   => to deliver SIGKILL
> Memory failure: 0xedbe201: Killing read_poison:22434 due to hardware memory corruption
>   => to deliver SIGBUS
>
> Signed-off-by: Jane Chu <jane.chu@oracle.com>
> Suggested-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Looks good, ignore the checkpatch warning about too long subject line,
looks appropriate to me:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>