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

Nitesh Narayan Lal
2019-02-11T16:24:31Z

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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. 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.

>
>>>
>>>> ---
>>>> 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

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