Re: [RFC PATCH 4/4] mm: Add merge page notifier

Michael S. Tsirkin
2019-02-11T17:41:21Z

On Mon, Feb 11, 2019 at 11:24:02AM -0500, Nitesh Narayan Lal wrote:
>
> On 2/11/19 9:17 AM, Michael S. Tsirkin wrote:
> > On Mon, Feb 11, 2019 at 08:30:03AM -0500, Nitesh Narayan Lal wrote:
> >> On 2/9/19 7:57 PM, Michael S. Tsirkin wrote:
> >>> On Mon, Feb 04, 2019 at 10:15:58AM -0800, Alexander Duyck wrote:
> >>>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>>>
> >>>> Because the implementation was limiting itself to only providing hints on
> >>>> pages huge TLB order sized or larger we introduced the possibility for free
> >>>> pages to slip past us because they are freed as something less then
> >>>> huge TLB in size and aggregated with buddies later.
> >>>>
> >>>> To address that I am adding a new call arch_merge_page which is called
> >>>> after __free_one_page has merged a pair of pages to create a higher order
> >>>> page. By doing this I am able to fill the gap and provide full coverage for
> >>>> all of the pages huge TLB order or larger.
> >>>>
> >>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>> Looks like this will be helpful whenever active free page
> >>> hints are added. So I think it's a good idea to
> >>> add a hook.
> >>>
> >>> However, could you split adding the hook to a separate
> >>> patch from the KVM hypercall based implementation?
> >>>
> >>> Then e.g. Nilal's patches could reuse it too.
> >> With the current design of my patch-set, if I use this hook to report
> >> free pages. I will end up making redundant hints for the same pfns.
> >>
> >> This is because the pages once freed by the host, are returned back to
> >> the buddy.
> > Suggestions on how you'd like to fix this? You do need this if
> > you introduce a size cut-off right?
>
> I do, there are two ways to go about it.
>
> One is to  use this and have a flag in the page structure indicating
> whether that page has been freed/used or not.

Not sure what do you mean. The refcount does this right?

> Though I am not sure if
> this will be acceptable upstream.
> Second is to find another place to invoke guest_free_page() post buddy
> merging.

Might be easier.

> >
> >>>
> >>>> ---
> >>>> arch/x86/include/asm/page.h | 12 ++++++++++++
> >>>> arch/x86/kernel/kvm.c | 28 ++++++++++++++++++++++++++++
> >>>> include/linux/gfp.h | 4 ++++
> >>>> mm/page_alloc.c | 2 ++
> >>>> 4 files changed, 46 insertions(+)
> >>>>
> >>>> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
> >>>> index 4487ad7a3385..9540a97c9997 100644
> >>>> --- a/arch/x86/include/asm/page.h
> >>>> +++ b/arch/x86/include/asm/page.h
> >>>> @@ -29,6 +29,18 @@ static inline void arch_free_page(struct page *page, unsigned int order)
> >>>> if (static_branch_unlikely(&pv_free_page_hint_enabled))
> >>>> __arch_free_page(page, order);
> >>>> }
> >>>> +
> >>>> +struct zone;
> >>>> +
> >>>> +#define HAVE_ARCH_MERGE_PAGE
> >>>> +void __arch_merge_page(struct zone *zone, struct page *page,
> >>>> + unsigned int order);
> >>>> +static inline void arch_merge_page(struct zone *zone, struct page *page,
> >>>> + unsigned int order)
> >>>> +{
> >>>> + if (static_branch_unlikely(&pv_free_page_hint_enabled))
> >>>> + __arch_merge_page(zone, page, order);
> >>>> +}
> >>>> #endif
> >>>>
> >>>> #include <linux/range.h>
> >>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> >>>> index 09c91641c36c..957bb4f427bb 100644
> >>>> --- a/arch/x86/kernel/kvm.c
> >>>> +++ b/arch/x86/kernel/kvm.c
> >>>> @@ -785,6 +785,34 @@ void __arch_free_page(struct page *page, unsigned int order)
> >>>> PAGE_SIZE << order);
> >>>> }
> >>>>
> >>>> +void __arch_merge_page(struct zone *zone, struct page *page,
> >>>> + unsigned int order)
> >>>> +{
> >>>> + /*
> >>>> + * The merging logic has merged a set of buddies up to the
> >>>> + * KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER. Since that is the case, take
> >>>> + * advantage of this moment to notify the hypervisor of the free
> >>>> + * memory.
> >>>> + */
> >>>> + if (order != KVM_PV_UNUSED_PAGE_HINT_MIN_ORDER)
> >>>> + return;
> >>>> +
> >>>> + /*
> >>>> + * Drop zone lock while processing the hypercall. This
> >>>> + * should be safe as the page has not yet been added
> >>>> + * to the buddy list as of yet and all the pages that
> >>>> + * were merged have had their buddy/guard flags cleared
> >>>> + * and their order reset to 0.
> >>>> + */
> >>>> + spin_unlock(&zone->lock);
> >>>> +
> >>>> + kvm_hypercall2(KVM_HC_UNUSED_PAGE_HINT, page_to_phys(page),
> >>>> + PAGE_SIZE << order);
> >>>> +
> >>>> + /* reacquire lock and resume freeing memory */
> >>>> + spin_lock(&zone->lock);
> >>>> +}
> >>>> +
> >>>> #ifdef CONFIG_PARAVIRT_SPINLOCKS
> >>>>
> >>>> /* Kick a cpu by its apicid. Used to wake up a halted vcpu */
> >>>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> >>>> index fdab7de7490d..4746d5560193 100644
> >>>> --- a/include/linux/gfp.h
> >>>> +++ b/include/linux/gfp.h
> >>>> @@ -459,6 +459,10 @@ static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
> >>>> #ifndef HAVE_ARCH_FREE_PAGE
> >>>> static inline void arch_free_page(struct page *page, int order) { }
> >>>> #endif
> >>>> +#ifndef HAVE_ARCH_MERGE_PAGE
> >>>> +static inline void
> >>>> +arch_merge_page(struct zone *zone, struct page *page, int order) { }
> >>>> +#endif
> >>>> #ifndef HAVE_ARCH_ALLOC_PAGE
> >>>> static inline void arch_alloc_page(struct page *page, int order) { }
> >>>> #endif
> >>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>>> index c954f8c1fbc4..7a1309b0b7c5 100644
> >>>> --- a/mm/page_alloc.c
> >>>> +++ b/mm/page_alloc.c
> >>>> @@ -913,6 +913,8 @@ static inline void __free_one_page(struct page *page,
> >>>> page = page + (combined_pfn - pfn);
> >>>> pfn = combined_pfn;
> >>>> order++;
> >>>> +
> >>>> + arch_merge_page(zone, page, order);
> >>>> }
> >>>> if (max_order < MAX_ORDER) {
> >>>> /* If we are here, it means order is >= pageblock_order.
> >> --
> >> Regards
> >> Nitesh
> >>
> >
> >
> --
> Regards
> Nitesh
>