linux-kernel - Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline

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Date:   Wed, 12 Jul 2023 21:10:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-doc@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check
 in offline_pages()

On 12.07.23 08:47, Anshuman Khandual wrote:
> 
> 
> On 7/11/23 23:10, David Hildenbrand wrote:
>> Let's update the documentation that any signal is sufficient, and
>> add a comment that not only checking for fatal signals is historical
>> baggage: changing it now could break existing user space. although
>> unlikely.
>>
>> For example, when an app provides a custom SIGALRM handler and triggers
>> memory offlining, the timeout cmd would no longer stop memory offlining,
>> because SIGALRM would no longer be considered a fatal signal.
>>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Jonathan Corbet <corbet@....net>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>   Documentation/admin-guide/mm/memory-hotplug.rst | 2 +-
>>   mm/memory_hotplug.c                             | 5 +++++
>>   2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
>> index 1b02fe5807cc..bd77841041af 100644
>> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
>> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
>> @@ -669,7 +669,7 @@ when still encountering permanently unmovable pages within ZONE_MOVABLE
>>   (-> BUG), memory offlining will keep retrying until it eventually succeeds.
>>   
>>   When offlining is triggered from user space, the offlining context can be
>> -terminated by sending a fatal signal. A timeout based offlining can easily be
>> +terminated by sending a signal. A timeout based offlining can easily be
>>   implemented via::
>>   
>>   	% timeout $TIMEOUT offline_block | failure_handling
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 3f231cf1b410..7cfd13c91568 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1843,6 +1843,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>>   	do {
>>   		pfn = start_pfn;
>>   		do {
>> +			/*
>> +			 * Historically we always checked for any signal and
>> +			 * can't limit it to fatal signals without eventually
>> +			 * breaking user space.> +			 */
> 
> Just curious, could 'signal type' to stop memory offline process be considered
> an ABI and cannot be changed in kernel ever if required ? Just wondering if an
> additional '!fatal_signal_pending()' check be introduced to warn about support
> being deprecated, before finally replacing it with fatal_signal_pending().

See my reply to Michal, while that would be doable it is probably not 
worth the effort, and we'd still have to stick with the existing 
handling for quite a while.

Thanks!

-- 
Cheers,

David / dhildenb