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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     Mika Penttil?? <mika.penttila@nextfour.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        "Mel Gorman" <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Zi Yan <ziy@nvidia.com>, Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Minchan Kim <minchan@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [RFC 2/3] mm: Add a new page flag PageLayzyFree() for MADV_FREE
References: <20200228033819.3857058-1-ying.huang@intel.com>
        <20200228033819.3857058-3-ying.huang@intel.com>
        <20200315081854.rcqlmfckeqrh7fbt@master>
        <92d4b0fe-f592-8da6-0282-2ea8a015b247@nextfour.com>
        <20200315122217.45mioaxzuulwvx2f@master>
Date:   Mon, 16 Mar 2020 09:21:34 +0800
In-Reply-To: <20200315122217.45mioaxzuulwvx2f@master> (Wei Yang's message of
        "Sun, 15 Mar 2020 12:22:17 +0000")
Message-ID: <87pnddrt5t.fsf@yhuang-dev.intel.com>
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Wei Yang <richard.weiyang@gmail.com> writes:

> On Sun, Mar 15, 2020 at 10:54:03AM +0200, Mika Penttil?? wrote:
>>
>>
>>On 15.3.2020 10.18, Wei Yang wrote:
>>> On Fri, Feb 28, 2020 at 11:38:18AM +0800, Huang, Ying wrote:
>>> > From: Huang Ying <ying.huang@intel.com>
>>> > 
>>> > Now !PageSwapBacked() is used as the flag for the pages freed lazily
>>> > via MADV_FREE.  This isn't obvious enough.  So Dave suggested to add a
>>> > new page flag for that to improve the code readability.
>>> I am confused with the usage of PageSwapBacked().
>>> 
>>> Previously I thought this flag means the page is swapin, set in
>>> swapin_readahead(). While I found page_add_new_anon_rmap() would set it too.
>>> This means every anon page would carry this flag. Then what is this flag
>>> means?
>>> 
>>> 
>>
>>But not all PageSwapBacked() pages are anon, like shmem.
>>
>
> Yes, while it looks shmem is the only exception.

Another exception is the pages freed lazily via MADV_FREE.

> I am still struggling to understand the meaning of this flag.

You can use `git blame` to find out the commit which introduces this
flag.  Which describes why this flag is introduced.

Best Regards,
Huang, Ying

>>
>>--Mika