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Date:   Tue, 11 Jul 2023 22:47:24 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-doc@vger.kernel.org, Oscar Salvador <osalvador@suse.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v1] mm/memory_hotplug: document the signal_pending()
 check in offline_pages()
Message-ID: <ZK2/3Kq8VlDwcCpc@dhcp22.suse.cz>
References: <20230711174050.603820-1-david@redhat.com>
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On Tue 11-07-23 19:40:50, 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.

Yes, and it is likely goot to mention here that this is an antipattern
for many other kernel operations like IO (e.g. write) but it is a long
term behavior that somebody might depend on and it is safer to reflect
the documentation to the realitity rather than other way around (which
would be imho better).

> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.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.
> +			 */
>  			if (signal_pending(current)) {
>  				ret = -EINTR;
>  				reason = "signal backoff";
> -- 
> 2.41.0

-- 
Michal Hocko
SUSE Labs