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

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.

>
>
>> ---
>> 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

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