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

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Message-ID: <20220108081340.3oi2z2rm3cbqozzt@master>
Date:   Sat, 8 Jan 2022 08:13:40 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Wei Yang <richard.weiyang@...il.com>, 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 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.

-- 
Wei Yang
Help you, Help me