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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Zi Yan <ziy@nvidia.com>, Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Minchan Kim" <minchan@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
References: <20200228033819.3857058-1-ying.huang@intel.com>
        <20200228034248.GE29971@bombadil.infradead.org>
        <87a7538977.fsf@yhuang-dev.intel.com>
        <edae2736-3239-0bdc-499c-560fc234c974@redhat.com>
        <871rqf850z.fsf@yhuang-dev.intel.com>
        <20200228095048.GK3771@dhcp22.suse.cz>
        <87d09u7sm2.fsf@yhuang-dev.intel.com>
        <8005e5a1-e2f2-1e57-ccb4-0cb9371b080d@redhat.com>
Date:   Tue, 03 Mar 2020 08:25:43 +0800
In-Reply-To: <8005e5a1-e2f2-1e57-ccb4-0cb9371b080d@redhat.com> (David
        Hildenbrand's message of "Mon, 2 Mar 2020 15:23:16 +0100")
Message-ID: <878ski708o.fsf@yhuang-dev.intel.com>
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David Hildenbrand <david@redhat.com> writes:

> On 02.03.20 15:12, Huang, Ying wrote:
>> Michal Hocko <mhocko@kernel.org> writes:
>> 
>>> On Fri 28-02-20 16:55:40, Huang, Ying wrote:
>>>> David Hildenbrand <david@redhat.com> writes:
>>> [...]
>>>>> E.g., free page reporting in QEMU wants to use MADV_FREE. The guest will
>>>>> report currently free pages to the hypervisor, which will MADV_FREE the
>>>>> reported memory. As long as there is no memory pressure, there is no
>>>>> need to actually free the pages. Once the guest reuses such a page, it
>>>>> could happen that there is still the old page and pulling in in a fresh
>>>>> (zeroed) page can be avoided.
>>>>>
>>>>> AFAIKs, after your change, we would get more pages discarded from our
>>>>> guest, resulting in more fresh (zeroed) pages having to be pulled in
>>>>> when a guest touches a reported free page again. But OTOH, page
>>>>> migration is speed up (avoiding to migrate these pages).
>>>>
>>>> Let's look at this problem in another perspective.  To migrate the
>>>> MADV_FREE pages of the QEMU process from the node A to the node B, we
>>>> need to free the original pages in the node A, and (maybe) allocate the
>>>> same number of pages in the node B.  So the question becomes
>>>>
>>>> - we may need to allocate some pages in the node B
>>>> - these pages may be accessed by the application or not
>>>> - we should allocate all these pages in advance or allocate them lazily
>>>>   when they are accessed.
>>>>
>>>> We thought the common philosophy in Linux kernel is to allocate lazily.
>>>
>>> The common philosophy is to cache as much as possible.
>> 
>> Yes.  This is another common philosophy.  And MADV_FREE pages is
>> different from caches such as the page caches because it has no valid
>> contents.
>
> Side note: It might contain valid content until discarded/zeroed out.
> E.g., an application could use a marker bit (e.g., first bit) to detect
> if the page still contains valid data or not. If the data is still
> marked valid, the content could be reuse immediately. Not sure if there
> is any such application, though :)

I don't think this is the typical use case.  But I admit that this is
possible.

Best Regards,
Huang, Ying