Re: [PATCH v2 4/7] mm,memory_hotplug: Enforce struct page size to be multiple of PMD

[David Hildenbrand]

On 01.03.21 11:06, Oscar Salvador wrote:
> On Thu, Feb 25, 2021 at 07:26:50PM +0100, David Hildenbrand wrote:
> > On 09.02.21 14:38, Oscar Salvador wrote:
> > > When struct page's size is not multiple of PMD, these do not get
> > > fully populated when adding sections, hence two sections will
> > > intersect the same the PMD. This goes against the vmemmap-per-device
> > > premise, so reject it if that is the case.
> > >
> > > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> > > ---
> > >    mm/memory_hotplug.c | 5 +++--
> > >    1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > > index 95695483a622..d3fb036d33fd 100644
> > > --- a/mm/memory_hotplug.c
> > > +++ b/mm/memory_hotplug.c
> > > @@ -57,10 +57,11 @@ static __meminit int memmap_on_memory_store(const char *val,
> > >    					    const struct kernel_param *kp)
> > >    {
> > >    	/*
> > > -	 * Fail silently in case we cannot enable it due to platform constraints.
> > > +	 * Fail silently in case we cannot enable it due to system constraints.
> > >    	 * User can always check whether it is enabled or not via /sys/module.
> > >    	 */
> > > -	if (!IS_ENABLED(CONFIG_ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE))
> > > +	if (!IS_ENABLED(CONFIG_ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE) ||
> > > +	    (PMD_SIZE % sizeof(struct page)))
> > >    		return 0;
> > >    	return param_set_bool(val, kp);
> >
> > Squash that into #1 - it's part of basic operation.
>
> Just to be sure we are on the same page.
> You mean to move this patch and patch#5 into the runtime check in
> mhp_supports_memmap_on_memory(), right?

Yes, then the compiler might even be able to figure out that what 
follows the check is dead code.

--
Thanks,

David / dhildenb