linux-kernel - Re: [PATCH] mm: remove offset check on page->compound

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Message-ID: <20220123013852.mm7eyn3z26v3hkc2@master>
Date:   Sun, 23 Jan 2022 01:38:52 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        peterz@...radead.org, vbabka@...e.cz, will@...nel.org,
        linyunsheng@...wei.com, aarcange@...hat.com, feng.tang@...el.com,
        ebiederm@...ssion.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove offset check on page->compound_head and
 folio->lru

On Sat, Jan 08, 2022 at 08:13:40AM +0000, Wei Yang wrote:
>On Sat, Jan 08, 2022 at 12:49:53AM +0000, Matthew Wilcox wrote:
>>On Fri, Jan 07, 2022 at 04:08:25PM -0800, Andrew Morton wrote:
>>> On Fri, 7 Jan 2022 22:11:20 +0000 Matthew Wilcox <willy@...radead.org> wrote:
>>> 
>>> > > Hi, Matthew
>>> > > 
>>> > > Would you mind sharing some insight on this check?
>>> > 
>>> > It's right there in the comments.
>>> 
>>> Well I can't figure out which comment you're referring to?
>>
>>         * WARNING: bit 0 of the first word is used for PageTail(). That
>>         * means the other users of this union MUST NOT use the bit to
>>         * avoid collision and false-positive PageTail().
>>
>
>I know this requirement on bit 0 of first word. But I don't see the connection
>between this and the check of page->compound_head and folio->lru.
>
>This is more like a internal requirement on struct page. There are 8 struct in
>this five words union. And this requirement apply to bit 0 of first word of
>all those five struct.
>
>To me, if folio has the same layout of page, folio meets this requirement. I
>still not catch the point why we need this check here.
>

Hi, Matthew

Are you back from vocation? If you could give more insight on this check, I
would be appreciated.

-- 
Wei Yang
Help you, Help me