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Subject: Re: [PATCH -V2] /proc/PID/smaps: Add PMD migration entry parsing
To:     Michal Hocko <mhocko@kernel.org>,
        "Huang, Ying" <ying.huang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Zi Yan <ziy@nvidia.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        =?UTF-8?B?Su+/vXLvv71tZSBHbGlzc2U=?= <jglisse@redhat.com>,
        Yang Shi <yang.shi@linux.alibaba.com>
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From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <a8b089b7-75a8-327c-0418-a5209af0571b@yandex-team.ru>
Date:   Thu, 2 Apr 2020 11:29:09 +0300
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On 02/04/2020 11.21, Michal Hocko wrote:
> On Thu 02-04-20 16:10:29, Huang, Ying wrote:
>> Michal Hocko <mhocko@kernel.org> writes:
>>
>>> On Thu 02-04-20 15:03:23, Huang, Ying wrote:
> [...]
>>>>> Could you explain why do we need this WARN_ON? I haven't really checked
>>>>> the swap support for THP but cannot we have normal swap pmd entries?
>>>>
>>>> I have some patches to add the swap pmd entry support, but they haven't
>>>> been merged yet.
>>>>
>>>> Similar checks are for all THP migration code paths, so I follow the
>>>> same style.
>>>
>>> I haven't checked other migration code paths but what is the reason to
>>> add the warning here? Even if this shouldn't happen, smaps is perfectly
>>> fine to ignore that situation, no?
>>
>> Yes. smaps itself is perfectly fine to ignore it.  I think this is used
>> to find bugs in other code paths such as THP migration related.
> 
> Please do not add new warnings without a good an strong reasons. As a
> matter of fact there are people running with panic_on_warn and each
> warning is fatal for them. Please also note that this is a user trigable
> path and that requires even more care.
> 

But this should not happen and if it does we'll never know without debug.
VM_WARN_ON checks something only if build with CONFIG_DEBUG_VM=y.

Anybody who runs debug kernels with panic_on_warn shouldn't expect much stability =)