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

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Message-ID: <20220124225531.26yyse52yo5x3fr5@master>
Date:   Mon, 24 Jan 2022 22:55:31 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        peterz@...radead.org, 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 Mon, Jan 24, 2022 at 11:30:10AM +0100, Vlastimil Babka wrote:
>On 1/23/22 02:38, Wei Yang wrote:
>> 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.
>
>I can offer my insight (which might be of course wrong). Ideally one day
>page.lru will be gone and only folio will be used for LRU pages. Then there
>won't be a  FOLIO_MATCH(lru, lru); and FOLIO_MATCH(compound_head, lru);
>won't appear to be redundant anymore. lru is list_head so two pointers and

Thanks for your comment.

I can't imagine the final result. If we would remove page.lru, we could remove
FOLIO_MATCH(lru, lru) and add FOLIO_MATCH(compound_head, lru) at that moment?

>thus valid pointers are aligned in such a way they can't accidentaly set the
>bit 0.
>

-- 
Wei Yang
Help you, Help me