Re: [PATCH v6 4/4] mm/hwpoison: introduce per-memory_block hwpoison counter

[Naoya Horiguchi]

On Thu, Oct 13, 2022 at 10:33:45AM +0200, Oscar Salvador wrote:
> On Fri, Oct 07, 2022 at 10:07:06AM +0900, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > 
> > Currently PageHWPoison flag does not behave well when experiencing memory
> > hotremove/hotplug.  Any data field in struct page is unreliable when the
> > associated memory is offlined, and the current mechanism can't tell whether
> > a memory block is onlined because a new memory devices is installed or
> > because previous failed offline operations are undone.  Especially if
> > there's a hwpoisoned memory, it's unclear what the best option is.
> > 
> > So introduce a new mechanism to make struct memory_block remember that
> > a memory block has hwpoisoned memory inside it. And make any online event
> > fail if the onlining memory block contains hwpoison.  struct memory_block
> > is freed and reallocated over ACPI-based hotremove/hotplug, but not over
> > sysfs-based hotremove/hotplug.  So the new counter can distinguish these
> > cases.
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> 
> I glanzed over it and looks good overall.
> Have a small question though:

Thank you for looking.

> 
> > @@ -864,6 +878,7 @@ void remove_memory_block_devices(unsigned long start, unsigned long size)
> >  		mem = find_memory_block_by_id(block_id);
> >  		if (WARN_ON_ONCE(!mem))
> >  			continue;
> > +		num_poisoned_pages_sub(-1UL, memblk_nr_poison(mem));
> 
> Why does num_poisoned_pages_sub() have to make this distinction (!-1 == -1)
> for the hot-remove stage?

The first argument is used to find memory_block including the given pfn.
And in the above context remove_memory_block_devices() already has the
pointer "mem", so recalcurating it looked to me not necessary.  Moreover,
this code is about to free the memory_block so updating the counter inside
it can be avoided.  This is just a tiny optimization, and there can be
better option.

Thanks,
Naoya Horiguchi