linux-kernel - Re: [PATCH v1 1/5] mm/memory_hotplug: check for fatal signals only in offline

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Message-ID: <d44b7a42-f89d-5bd5-9f29-e8643f6ee17d@redhat.com>
Date:   Tue, 27 Jun 2023 15:28:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Oscar Salvador <osalvador@...e.de>,
        Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH v1 1/5] mm/memory_hotplug: check for fatal signals only in
 offline_pages()

On 27.06.23 14:34, Michal Hocko wrote:
> On Tue 27-06-23 13:22:16, David Hildenbrand wrote:
>> Let's check for fatal signals only. That looks cleaner and still keeps
>> the documented use case for manual user-space triggered memory offlining
>> working. From Documentation/admin-guide/mm/memory-hotplug.rst:
>>
>> 	% timeout $TIMEOUT offline_block | failure_handling
>>
>> In fact, we even document there: "the offlining context can be terminated
>> by sending a fatal signal".
> 
> We should be fixing documentation instead. This could break users who do
> have a SIGALRM signal hander installed.

You mean because timeout will send a SIGALRM, which is not considered 
fatal in case a signal handler is installed?

At least the "traditional" tools I am aware of don't set a timeout at 
all (crossing fingers that they never end up stuck):
* chmem
* QEMU guest agent
* powerpc-utils

libdaxctl also doesn't seem to implement an easy-to-spot timeout for 
memory offlining, but it also doesn't configure SIGALRM.


Of course, that doesn't mean that there isn't somewhere a program that 
does that; I merely assume that it would be pretty unlikely to find such 
a program.

But no strong opinion: we can also keep it like that, update the doc and 
add a comment why this one here is different than most other signal 
backoff checks.


Thanks!

-- 
Cheers,

David / dhildenb