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

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Message-ID: <ZJrtO105xKpzbR9g@dhcp22.suse.cz>
Date:   Tue, 27 Jun 2023 16:07:55 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.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 Tue 27-06-23 15:28:29, David Hildenbrand wrote:
> 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?

Correct.

> 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.

Well, the existing signal handling approach is there for way too long to
be sure. I personally would prefer fatal_signal_pending as that reflects
more what we do elsewhere but here we are. Historical baggage...
-- 
Michal Hocko
SUSE Labs