Re: [PATCH] mm/sparse: reset section's mem_map when fully deactivated

[Pingfan Liu <kernelfans@gmail.com>]

On Thu, Jan 16, 2020 at 4:25 PM Baoquan He <bhe@redhat.com> wrote:
>
> On 01/16/20 at 09:14am, David Hildenbrand wrote:
> > On 16.01.20 09:06, David Hildenbrand wrote:
> > > On 16.01.20 04:01, Pingfan Liu wrote:
> > >> When fully deactivated, it is meaningless to keep the value of a section's
> > >> mem_map. And its mem_map will be reassigned during re-added.
> > >>
> > >> Beside this, it breaks the user space tool "makedumpfile", which makes
> > >> assumption that a hot-removed section having mem_map as NULL.
> > >>
> > >> The bug can be reproduced on IBM POWERVM by "drmgr -c mem -r -q 5" ,
> > >> trigger a crash, and save vmcore by makedumpfile
> > >
> > > Are you using an up-to-date makedumfile and did kdump.service properly
> > > get reloaded on the udev events? I remember that this works.
> > >
> > > makedumpfile will not dump memory sections that a) are not marked
> > > offline (SECTION_IS_ONLINE) - after offlining b) are not part of an
> > > iomem resource - after memory unplug.
>
> Makedumpfile seems to only check SECTION_MARKED_PRESENT. Then the NULL
> memmap will fail vmcore dumping, I guess.
makedumpfile.c / validate_mem_section() has some trick, so it will not fail.
>
> > >
> > >
> > > The current code makes sure that sparse_decode_mem_map() will return NULL.
> > >
> >
> > ... but it's only used at this very place. I think we should add a
> > Fixes: tag, although this might be fixed as well in makedumpfile (so
> > people are aware that patch broke something)
>
> Agree, it's worth fixing it too in makedumpfile side to enhance.
>
Yeah, I also agree :)