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Date:   Mon, 24 Jan 2022 11:30:10 +0100
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Subject: Re: [PATCH] mm: remove offset check on page->compound_head and
 folio->lru
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To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        peterz@infradead.org, will@kernel.org, linyunsheng@huawei.com,
        aarcange@redhat.com, feng.tang@intel.com, ebiederm@xmission.com,
        linux-kernel@vger.kernel.org
References: <20220106235254.19190-1-richard.weiyang@gmail.com>
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 <20220108081340.3oi2z2rm3cbqozzt@master>
 <20220123013852.mm7eyn3z26v3hkc2@master>
From:   Vlastimil Babka <vbabka@suse.cz>
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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@infradead.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
thus valid pointers are aligned in such a way they can't accidentaly set the
bit 0.