Re: [RFC PATCH 3/3] mm: support free hugepage pre zero out

[David Hildenbrand <david@redhat.com>]

On 22.12.20 09:31, David Hildenbrand wrote:
> On 22.12.20 08:49, Liang Li wrote:
>> This patch add support of pre zero out free hugepage, we can use
>> this feature to speed up page population and page fault handing.
>>
>> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Dave Hansen <dave.hansen@intel.com>
>> Cc: David Hildenbrand <david@redhat.com>  
>> Cc: Michal Hocko <mhocko@suse.com> 
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Alex Williamson <alex.williamson@redhat.com>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Cc: Jason Wang <jasowang@redhat.com>
>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>> Cc: Liang Li <liliang324@gmail.com>
>> Signed-off-by: Liang Li <liliangleo@didiglobal.com>
>> ---
>>  mm/page_prezero.c | 17 +++++++++++++++++
>>  1 file changed, 17 insertions(+)
>>
>> diff --git a/mm/page_prezero.c b/mm/page_prezero.c
>> index c8ce720bfc54..dff4e0adf402 100644
>> --- a/mm/page_prezero.c
>> +++ b/mm/page_prezero.c
>> @@ -26,6 +26,7 @@ static unsigned long delay_millisecs = 1000;
>>  static unsigned long zeropage_enable __read_mostly;
>>  static DEFINE_MUTEX(kzeropaged_mutex);
>>  static struct page_reporting_dev_info zero_page_dev_info;
>> +static struct page_reporting_dev_info zero_hugepage_dev_info;
>>  
>>  inline void clear_zero_page_flag(struct page *page, int order)
>>  {
>> @@ -69,9 +70,17 @@ static int start_kzeropaged(void)
>>  		zero_page_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
>>  
>>  		err = page_reporting_register(&zero_page_dev_info);
>> +
>> +		zero_hugepage_dev_info.report = zero_free_pages;
>> +		zero_hugepage_dev_info.mini_order = mini_page_order;
>> +		zero_hugepage_dev_info.batch_size = batch_size;
>> +		zero_hugepage_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
>> +
>> +		err |= hugepage_reporting_register(&zero_hugepage_dev_info);
>>  		pr_info("Zero page enabled\n");
>>  	} else {
>>  		page_reporting_unregister(&zero_page_dev_info);
>> +		hugepage_reporting_unregister(&zero_hugepage_dev_info);
>>  		pr_info("Zero page disabled\n");
>>  	}
>>  
>> @@ -90,7 +99,15 @@ static int restart_kzeropaged(void)
>>  		zero_page_dev_info.batch_size = batch_size;
>>  		zero_page_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
>>  
>> +		hugepage_reporting_unregister(&zero_hugepage_dev_info);
>> +
>> +		zero_hugepage_dev_info.report = zero_free_pages;
>> +		zero_hugepage_dev_info.mini_order = mini_page_order;
>> +		zero_hugepage_dev_info.batch_size = batch_size;
>> +		zero_hugepage_dev_info.delay_jiffies = msecs_to_jiffies(delay_millisecs);
>> +
>>  		err = page_reporting_register(&zero_page_dev_info);
>> +		err |= hugepage_reporting_register(&zero_hugepage_dev_info);
>>  		pr_info("Zero page enabled\n");
>>  	}
>>  
>>
> 
> Free page reporting in virtio-balloon doesn't give you any guarantees
> regarding zeroing of pages. Take a look at the QEMU implementation -
> e.g., with vfio all reports are simply ignored.
> 
> Also, I am not sure if mangling such details ("zeroing of pages") into
> the page reporting infrastructure is a good idea.
> 

Oh, now I get what you are doing here, you rely on zero_free_pages of
your other patch series and are not relying on virtio-balloon free page
reporting to do the zeroing.

You really should have mentioned that this patch series relies on the
other one and in which way.

-- 
Thanks,

David / dhildenb