Re: [PATCH 2/3] KVM: Guest page hinting functionality

[Nitesh Narayan Lal <nilal@redhat.com>]

[-- Attachment #1.1: Type: text/plain, Size: 9738 bytes --]

Hi Pankaj,

Thanks for your comments, please find my response inline.


On 08/02/2017 03:01 AM, Pankaj Gupta wrote:
> Hi Nitesh,
>
> I tried to look at the code. 
> Some minor comments inline.
>
> Thanks,
> Pankaj
>
>> This patch adds the guest implementation in order to maintain the list of
>> pages which are freed by the guest and are not reused. To avoid any
>> reallocation it includes seqlock once the list is completely filled.
>> Though it doesn't carries the hypercall related changes.
>>
>> Signed-off-by: Nitesh Narayan Lal <nilal@redhat.com>
>> ---
>>  virt/kvm/page_hinting.c | 246
>>  +++++++++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 244 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/page_hinting.c b/virt/kvm/page_hinting.c
>> index 39d2b1d..cfdc513 100644
>> --- a/virt/kvm/page_hinting.c
>> +++ b/virt/kvm/page_hinting.c
>> @@ -3,7 +3,7 @@
>>  #include <linux/page_ref.h>
>>  #include <linux/kvm_host.h>
>>  #include <linux/sort.h>
>> -
>> +#include <linux/kernel.h>
>>  #include <trace/events/kvm.h>
>>  
>>  #define MAX_FGPT_ENTRIES	1000
>> @@ -33,14 +33,256 @@ struct hypervisor_pages {
>>  	unsigned int pages;
>>  };
>>  
>> +static __cacheline_aligned_in_smp DEFINE_SEQLOCK(guest_page_lock);
>>  DEFINE_PER_CPU(struct kvm_free_pages [MAX_FGPT_ENTRIES], kvm_pt);
>>  DEFINE_PER_CPU(int, kvm_pt_idx);
>> -struct hypervisor_pages hypervisor_pagelist[MAX_FGPT_ENTRIES];
>> +struct hypervisor_pages hypervisor_pagelist[MAX_FGPT_ENTRIES - 1];
>> +
>> +static void empty_hyperlist(void)
>> +{
>> +	int i = 0;
>> +
>> +	while (i < MAX_FGPT_ENTRIES - 1) {
>       MAX_FGPT_ENTRIES in-place of 'MAX_FGPT_ENTRIES - 1' here
>       and at similar other places?
This is because CPU local list has a total of 1000 entries
(MAX_FGPT_ENTRIES) where as CPU global list has 999 entries. If you see
the arch_free_page_slowpath() and consider a situation where there are
1000 entries of singly allocated free pages in cpu-local list i.e., none
of them are re-allocated. While adding them to the cpu global list when
cpu local list index reaches to 1000 the outer loop will terminate due
to which cpu global list index will never reach to 1000 and
compress_hyperlist()/make_hypercall() will never be called.
>
>> +		hypervisor_pagelist[i].pfn = 0;
>> +		hypervisor_pagelist[i].pages = 0;
>> +		i++;
>> +	}
>> +}
>> +
>> +static void make_hypercall(void)
>> +{
>> +	/*
>> +	 * Dummy function: Tobe filled later.
>> +	 */
>> +	empty_hyperlist();
>> +}
>> +
>> +static int sort_pfn(const void *a1, const void *b1)
>> +{
>> +	const struct hypervisor_pages *a = a1;
>> +	const struct hypervisor_pages *b = b1;
>> +
>> +	if (a->pfn > b->pfn)
>> +		return 1;
>> +
>> +	if (a->pfn < b->pfn)
>> +		return -1;
>> +
>> +	return 0;
>> +}
>> +
>> +static int pack_hyperlist(void)
>> +{
>> +	int i = 0, j = 0;
>> +
>> +	while (i < MAX_FGPT_ENTRIES  - 1) {
>> +		if (hypervisor_pagelist[i].pfn != 0) {
>    no need to copy values at same index if non-zero
I agree, I will fix it in the next version.
>                    
>
>> +			hypervisor_pagelist[j].pfn =
>> +					hypervisor_pagelist[i].pfn;
>> +			hypervisor_pagelist[j].pages =
>> +					hypervisor_pagelist[i].pages;
>> +			j++;
>> +		}
>> +		i++;
>> +	}
>> +	i = j;
>> +	while (j < MAX_FGPT_ENTRIES - 1) {
>> +		hypervisor_pagelist[j].pfn = 0;
>> +		hypervisor_pagelist[j].pages = 0;
>> +		j++;
>> +	}
>> +	return i;
>> +}
>> +
>> +int compress_hyperlist(void)
>> +{
>> +	int i = 0, j = 1, merge_counter = 0, ret = 0;
>> +
>> +	sort(hypervisor_pagelist, MAX_FGPT_ENTRIES - 1,
>> +	     sizeof(struct hypervisor_pages), sort_pfn, NULL);
>> +	while (i < MAX_FGPT_ENTRIES - 1 && j < MAX_FGPT_ENTRIES - 1) {
>> +		unsigned long pfni = hypervisor_pagelist[i].pfn;
>> +		unsigned int pagesi = hypervisor_pagelist[i].pages;
>> +		unsigned long pfnj = hypervisor_pagelist[j].pfn;
>> +		unsigned int pagesj = hypervisor_pagelist[j].pages;
>> +
>> +		if (pfnj <= pfni) {
>> +			if (((pfnj + pagesj - 1) <= (pfni + pagesi - 1)) &&
>> +			    ((pfnj + pagesj - 1) >= (pfni - 1))) {
>> +				hypervisor_pagelist[i].pfn = pfnj;
>> +				hypervisor_pagelist[i].pages += pfni - pfnj;
>> +				hypervisor_pagelist[j].pfn = 0;
>> +				hypervisor_pagelist[j].pages = 0;
>> +				j++;
>> +				merge_counter++;
>> +				continue;
>> +			} else if ((pfnj + pagesj - 1) > (pfni + pagesi - 1)) {
>> +				hypervisor_pagelist[i].pfn = pfnj;
>> +				hypervisor_pagelist[i].pages = pagesj;
>> +				hypervisor_pagelist[j].pfn = 0;
>> +				hypervisor_pagelist[j].pages = 0;
>> +				j++;
>> +				merge_counter++;
>> +				continue;
>> +			}
>> +		}
>> +		if (pfnj > pfni) {
>              
>           else if here
I will fix this in the next version.
>> +			if ((pfnj + pagesj - 1) > (pfni + pagesi - 1) &&
>> +			    (pfnj <= pfni + pagesi)) {
>> +				hypervisor_pagelist[i].pages +=
>> +						(pfnj + pagesj - 1) -
>> +						(pfni + pagesi - 1);
>> +				hypervisor_pagelist[j].pfn = 0;
>> +				hypervisor_pagelist[j].pages = 0;
>> +				j++;
>> +				merge_counter++;
>> +				continue;
>> +			}
>> +			if ((pfnj + pagesj - 1) <= (pfni + pagesi - 1)) {
>> +				hypervisor_pagelist[j].pfn = 0;
>> +				hypervisor_pagelist[j].pages = 0;
>> +				j++;
>> +				merge_counter++;
>> +				continue;
>> +			}
>> +		}
>> +		i = j;
>> +		j++;
>> +	}
>> +	if (merge_counter != 0)
>> +		ret = pack_hyperlist() - 1;
>> +	else
>> +		ret = MAX_FGPT_ENTRIES - 1;
>> +	return ret;
>> +}
>> +
>> +void copy_hyperlist(int hyper_idx)
>> +{
>> +	int *idx = &get_cpu_var(kvm_pt_idx);
>> +	struct kvm_free_pages *free_page_obj;
>> +	int i = 0;
>> +
>> +	free_page_obj = &get_cpu_var(kvm_pt)[0];
>> +	while (i < hyper_idx) {
>> +		free_page_obj[*idx].pfn = hypervisor_pagelist[i].pfn;
>> +		free_page_obj[*idx].pages = hypervisor_pagelist[i].pages;
>> +		*idx += 1;
>> +		i++;
>> +	}
>> +	empty_hyperlist();
>> +	put_cpu_var(kvm_pt);
>> +	put_cpu_var(kvm_pt_idx);
>> +}
>> +
>> +/*
>> + * arch_free_page_slowpath() - This function adds the guest free page
>> entries
>> + * to hypervisor_pages list and also ensures defragmentation prior to
>> addition
>> + * if it is present with any entry of the kvm_free_pages list.
>> + */
>> +void arch_free_page_slowpath(void)
>> +{
>> +	int idx = 0;
>> +	int hyper_idx = -1;
>> +	int *kvm_idx = &get_cpu_var(kvm_pt_idx);
>> +	struct kvm_free_pages *free_page_obj = &get_cpu_var(kvm_pt)[0];
>> +
>> +	write_seqlock(&guest_page_lock);
>> +	while (idx < MAX_FGPT_ENTRIES) {
>> +		unsigned long pfn = free_page_obj[idx].pfn;
>> +		unsigned long pfn_end = free_page_obj[idx].pfn +
>> +					free_page_obj[idx].pages - 1;
>> +		bool prev_free = false;
>> +
>> +		while (pfn <= pfn_end) {
>> +			struct page *p = pfn_to_page(pfn);
>> +
>> +			if (PageCompound(p)) {
>> +				struct page *head_page = compound_head(p);
>> +				unsigned long head_pfn = page_to_pfn(head_page);
>> +				unsigned int alloc_pages =
>> +					1 << compound_order(head_page);
>> +
>> +				pfn = head_pfn + alloc_pages;
>> +				prev_free = false;
>> +				continue;
>> +			}
>> +			if (page_ref_count(p)) {
>> +				pfn++;
>> +				prev_free = false;
>> +				continue;
>> +			}
>> +			/*
>> +			 * The page is free so add it to the list and free the
>> +			 * hypervisor_pagelist if required.
>> +			 */
>> +			if (!prev_free) {
>> +				hyper_idx++;
>> +				if (hyper_idx == MAX_FGPT_ENTRIES - 1) {
>> +					hyper_idx =  compress_hyperlist();
>> +					if (hyper_idx >=
>> +					    HYPERLIST_THRESHOLD - 1) {
>> +						make_hypercall();
>> +						hyper_idx = 0;
>> +					}
>> +				}
>> +				hypervisor_pagelist[hyper_idx].pfn = pfn;
>> +				hypervisor_pagelist[hyper_idx].pages = 1;
>> +				/*
>> +				 * If the next contiguous page is free, it can
>> +				 * be added to this same entry.
>> +				 */
>> +				prev_free = true;
>> +			} else {
>> +				/*
>> +				 * Multiple adjacent free pages
>> +				 */
>> +				hypervisor_pagelist[hyper_idx].pages++;
>> +			}
>> +			pfn++;
>> +		}
>> +		free_page_obj[idx].pfn = 0;
>> +		free_page_obj[idx].pages = 0;
>> +		idx++;
>> +	}
>> +	*kvm_idx = 0;
>> +	put_cpu_var(kvm_pt);
>> +	put_cpu_var(kvm_pt_idx);
>> +	write_sequnlock(&guest_page_lock);
>> +}
>>  
>>  void arch_alloc_page(struct page *page, int order)
>>  {
>> +	unsigned int seq;
>> +
>> +	/*
>> +	 * arch_free_page will acquire the lock once the list carrying guest
>> +	 * free pages is full and a hypercall will be made. Until complete free
>> +	 * page list is traversed no further allocaiton will be allowed.
>> +	 */
>> +	do {
>> +		seq = read_seqbegin(&guest_page_lock);
>> +	} while (read_seqretry(&guest_page_lock, seq));
>>  }
>>  
>>  void arch_free_page(struct page *page, int order)
>>  {
>> +	int *free_page_idx = &get_cpu_var(kvm_pt_idx);
>> +	struct kvm_free_pages *free_page_obj;
>> +	unsigned long flags;
>> +
>> +	/*
>> +	 * use of global variables may trigger a race condition between irq and
>> +	 * process context causing unwanted overwrites. This will be replaced
>> +	 * with a better solution to prevent such race conditions.
>> +	 */
>> +	local_irq_save(flags);
>> +	free_page_obj = &get_cpu_var(kvm_pt)[0];
>> +	free_page_obj[*free_page_idx].pfn = page_to_pfn(page);
>> +	free_page_obj[*free_page_idx].pages = 1 << order;
>> +	*free_page_idx += 1;
>> +	if (*free_page_idx == MAX_FGPT_ENTRIES)
>> +		arch_free_page_slowpath();
>> +	put_cpu_var(kvm_pt);
>> +	put_cpu_var(kvm_pt_idx);
>> +	local_irq_restore(flags);
>>  }
>> --
>> 2.9.4
>>
>>

-- 
Regards
Nitesh


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 819 bytes --]