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

[Michal Hocko]

On Fri 24-11-17 14:49:17, Andrea Reale wrote:
> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Maciej Bielski <m.bielski@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >  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. 

remove_memory failure is basically impossible to handle AFAIR. The
original code to BUG in remove_memory is ugly as hell and we do not want
to spread that out of that function. Instead we really want to get rid
of it.
-- 
Michal Hocko
SUSE Labs