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

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Message-ID: <a0bee19a-0703-54b1-2903-60383ab7da64@redhat.com>
Date:   Tue, 22 Dec 2020 09:49:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Liang Li <liliangleo@...iglobal.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Liang Li <liliang324@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org
Subject: Re: [RFC PATCH 3/3] mm: support free hugepage pre zero out

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@...ux.intel.com>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Cc: Dan Williams <dan.j.williams@...el.com>
>> Cc: Dave Hansen <dave.hansen@...el.com>
>> Cc: David Hildenbrand <david@...hat.com>  
>> Cc: Michal Hocko <mhocko@...e.com> 
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Alex Williamson <alex.williamson@...hat.com>
>> Cc: Michael S. Tsirkin <mst@...hat.com>
>> Cc: Jason Wang <jasowang@...hat.com>
>> Cc: Mike Kravetz <mike.kravetz@...cle.com>
>> Cc: Liang Li <liliang324@...il.com>
>> Signed-off-by: Liang Li <liliangleo@...iglobal.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