On Fri, Nov 24, 2017 at 07:17:41PM +0100, Michal Hocko wrote:
> On Fri 24-11-17 15:54:59, Andrea Reale wrote:
> > On Fri 24 Nov 2017, 16:43, Michal Hocko wrote:
> > > 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.
> >
> > Today, BUG() is called even in the simple case where remove fails
> > because the section we are removing is not offline.
>
> You cannot hotremove memory which is still online. This is what caller
> should enforce. This is too late to handle the failure. At least for
> ACPI.
>
The logic in acpi_scan_hot_remove() calls memory_subsys_offline(). If
there doesn't have any error returns by memory_subsys_offline, then ACPI
assumes all devices are offlined by subsystem (memory subsystem in this case).
Then system moves to remove stage, ACPI calls acpi_memory_device_remove().
Here
> > I cannot see any need to
> > BUG() in such a case: an error code seems more than sufficient to me.
>
> I do not rememeber details but AFAIR ACPI is in a deferred (kworker)
> context here and cannot simply communicate error code down the road.
> I agree that we should be able to simply return an error but what is the
> actual error condition that might happen here?
>
Currently acpi_bus_trim() didn't handle any return error. If subsystem
returns error, then ACPI can only interrupt hot-remove process.
> > This is why this patch removes the BUG() call when the "offline" check
> > fails from the generic code.
>
> As I've said we should simply get rid of BUG rather than move it around.
>
As I remember that the original BUG() helped us to find out a bug about the
offline state doesn't sync between memblock device with memory state.
Something likes:
mem->dev.offline != (mem->state == MEM_OFFLINE)
So, the BUG() is useful to capture bug about state sync between device object
and subsystem object.
Thanks
Joey Lee
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