Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

[Michal Hocko]

On Thu 11-10-18 07:59:32, Arun KS wrote:
> On 2018-10-10 23:03, Michal Hocko wrote:
> > On Wed 10-10-18 22:26:41, Arun KS wrote:
> > > On 2018-10-10 21:00, Vlastimil Babka wrote:
> > > > On 10/5/18 10:10 AM, Arun KS wrote:
> > > > > When free pages are done with higher order, time spend on
> > > > > coalescing pages by buddy allocator can be reduced. With
> > > > > section size of 256MB, hot add latency of a single section
> > > > > shows improvement from 50-60 ms to less than 1 ms, hence
> > > > > improving the hot add latency by 60%. Modify external
> > > > > providers of online callback to align with the change.
> > > > >
> > > > > Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>
> > > >
> > > > [...]
> > > >
> > > > > @@ -655,26 +655,44 @@ void __online_page_free(struct page *page)
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(__online_page_free);
> > > > >
> > > > > -static void generic_online_page(struct page *page)
> > > > > +static int generic_online_page(struct page *page, unsigned int order)
> > > > >  {
> > > > > -	__online_page_set_limits(page);
> > > >
> > > > This is now not called anymore, although the xen/hv variants still do
> > > > it. The function seems empty these days, maybe remove it as a followup
> > > > cleanup?
> > > >
> > > > > -	__online_page_increment_counters(page);
> > > > > -	__online_page_free(page);
> > > > > +	__free_pages_core(page, order);
> > > > > +	totalram_pages += (1UL << order);
> > > > > +#ifdef CONFIG_HIGHMEM
> > > > > +	if (PageHighMem(page))
> > > > > +		totalhigh_pages += (1UL << order);
> > > > > +#endif
> > > >
> > > > __online_page_increment_counters() would have used
> > > > adjust_managed_page_count() which would do the changes under
> > > > managed_page_count_lock. Are we safe without the lock? If yes, there
> > > > should perhaps be a comment explaining why.
> > > 
> > > Looks unsafe without managed_page_count_lock.
> > 
> > Why does it matter actually? We cannot online/offline memory in
> > parallel. This is not the case for the boot where we initialize memory
> > in parallel on multiple nodes. So this seems to be safe currently unless
> > I am missing something. A comment explaining that would be helpful
> > though.
> 
> Other main callers of adjust_manage_page_count(),
> 
> static inline void free_reserved_page(struct page *page)
> {
>         __free_reserved_page(page);
>         adjust_managed_page_count(page, 1);
> }
> 
> static inline void mark_page_reserved(struct page *page)
> {
>         SetPageReserved(page);
>         adjust_managed_page_count(page, -1);
> }
> 
> Won't they race with memory hotplug?
> 
> Few more,
> ./drivers/xen/balloon.c:519:            adjust_managed_page_count(page, -1);
> ./drivers/virtio/virtio_balloon.c:175:  adjust_managed_page_count(page, -1);
> ./drivers/virtio/virtio_balloon.c:196:  adjust_managed_page_count(page, 1);
> ./mm/hugetlb.c:2158:                    adjust_managed_page_count(page, 1 <<
> h->order);

They can, and I have missed those.

-- 
Michal Hocko
SUSE Labs