linux-kernel - Re: [RFC 2/3] mm: Add a new page flag PageLayzyFree() for MADV

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Date:   Mon, 16 Mar 2020 22:38:14 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Mika Penttil?? <mika.penttila@...tfour.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Zi Yan <ziy@...dia.com>, Michal Hocko <mhocko@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC 2/3] mm: Add a new page flag PageLayzyFree() for MADV_FREE

On Mon, Mar 16, 2020 at 09:21:34AM +0800, Huang, Ying wrote:
>Wei Yang <richard.weiyang@...il.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@...el.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.

Thanks, I see the purpose is to distinguish a page:

   a) file backed
   b) or otheres

This sound more clear.

And now this flag is also used for MADV_FREE, which sounds a little abuse.
This is the purpose of this patch to make a dedicate flag for MADV_FREE.

BTW, the name, swapbacked, is a little misleading. Maybe just to me. But I
can't come up with better naming. :-)

>
>Best Regards,
>Huang, Ying
>
>>>
>>>--Mika

-- 
Wei Yang
Help you, Help me