linux-kernel - Re: [PATCH] mm/debug: PageAnon() is true for PageKsm() pages

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Message-ID: <2019111510320597353014@gmail.com>
Date:   Fri, 15 Nov 2019 10:32:07 +0800
From:   "lixinhai.lxh@...il.com" <lixinhai.lxh@...il.com>
To:     "Michal Hocko" <mhocko@...nel.org>,
        "Ralph Campbell" <rcampbell@...dia.com>
Cc:     akpm <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/debug: PageAnon() is true for PageKsm() pages

On 2019-11-14 at 21:59 Michal Hocko wrote:
>On Tue 12-11-19 16:06:51, Ralph Campbell wrote:
>> PageAnon() and PageKsm() use the low two bits of the page->mapping pointer
>> to indicate the page type. PageAnon() only checks the LSB while PageKsm()
>> checks the least significant 2 bits are equal to 3. Therefore, PageAnon()
>> is true for KSM pages.
>> __dump_page() incorrectly will never print "ksm" because it checks
>> PageAnon() first. Fix this by checking PageKsm() first.
>
>Thanks for spotting this
>
>Fixes: 1c6fb1d89e73 ("mm: print more information about mapping in __dump_page")
>
>> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
>
>Acked-by: Michal Hocko <mhocko@...e.com>
>
The four values from those two lowest bits have different meaning, so PageKsm is 
true does not mean we can consider PageAnon is also true or PageMovable is also 
true.
Improve PageAnon() would be better, so users of those APIs do not need to 
remember this implict checking sequence at other places.

- Xinhai