Re: [PATCH v2 2/5] mm: memory_hotplug: Remove assumption on memory state before hotremove

[Andrea Reale <ar@linux.vnet.ibm.com>]

Hi Rafael,

On Fri 24 Nov 2017, 15:39, Rafael J. Wysocki wrote:
> On Fri, Nov 24, 2017 at 11:22 AM, Andrea Reale <ar@linux.vnet.ibm.com> wrote:
> > Resending the patch adding linux-acpi in CC, as suggested by Rafael.
> > Everyone else: apologies for the noise.
> >
> > Commit 242831eb15a0 ("Memory hotplug / ACPI: Simplify memory removal")
> > introduced an assumption whereas when control
> > reaches remove_memory the corresponding memory has been already
> > offlined. In that case, the acpi_memhotplug was making sure that
> > the assumption held.
> > This assumption, however, is not necessarily true if offlining
> > and removal are not done by the same "controller" (for example,
> > when first offlining via sysfs).
> >
> > Removing this assumption for the generic remove_memory code
> > and moving it in the specific acpi_memhotplug code. This is
> > a dependency for the software-aided arm64 offlining and removal
> > process.
> >
> > Signed-off-by: Andrea Reale <ar@linux.vnet.ibm.com>
> > Signed-off-by: Maciej Bielski <m.bielski@linux.vnet.ibm.com>
> > ---
> >  drivers/acpi/acpi_memhotplug.c |  2 +-
> >  include/linux/memory_hotplug.h |  9 ++++++---
> >  mm/memory_hotplug.c            | 13 +++++++++----
> >  3 files changed, 16 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c
> > index 6b0d3ef..b0126a0 100644
> > --- a/drivers/acpi/acpi_memhotplug.c
> > +++ b/drivers/acpi/acpi_memhotplug.c
> > @@ -282,7 +282,7 @@ static void acpi_memory_remove_memory(struct acpi_memory_device *mem_device)
> >                         nid = memory_add_physaddr_to_nid(info->start_addr);
> >
> >                 acpi_unbind_memory_blocks(info);
> > -               remove_memory(nid, info->start_addr, info->length);
> > +               BUG_ON(remove_memory(nid, info->start_addr, info->length));
> 
> Why does this have to be BUG_ON()?  Is it really necessary to kill the
> system here?

Actually, I hoped you would help me understand that: that BUG() call was introduced
by yourself in Commit 242831eb15a0 ("Memory hotplug / ACPI: Simplify memory removal")
in memory_hoptlug.c:remove_memory()). 

Just reading at that commit my understanding was that you were assuming
that acpi_memory_remove_memory() have already done the job of offlining
the target memory, so there would be a bug if that wasn't the case.

In my case, that assumption did not hold and I found that it might not
hold for other platforms that do not use ACPI. In fact, the purpose of
this patch is to move this assumption out of the generic hotplug code
and move it to ACPI code where it originated. 

Thanks,
Andrea

> If it is, please add a comment describing why continuing is not an option here.
> 
> >                 list_del(&info->list);
> >                 kfree(info);
> >         }
> 
> Thanks,
> Rafael
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