Re: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

[David Hildenbrand]

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@xxxxxxxx>
> > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   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