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Date:   Tue, 25 Jan 2022 11:11:40 +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>
 <Yde6hZ41agqa2zs3@casper.infradead.org>
 <20220107134059.flxr2hcd6ilb6vt7@master>
 <Ydi6iMbSZ/FewYPT@casper.infradead.org>
 <20220107160825.13c71fdd871d7d5611d116b9@linux-foundation.org>
 <YdjfsbAR0UlwyC6b@casper.infradead.org>
 <20220108081340.3oi2z2rm3cbqozzt@master>
 <20220123013852.mm7eyn3z26v3hkc2@master>
 <93c48e68-2266-72ee-0763-65805b94c968@suse.cz>
 <20220124225531.26yyse52yo5x3fr5@master>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20220124225531.26yyse52yo5x3fr5@master>
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On 1/24/22 23:55, Wei Yang wrote:
> 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:
>>>>
>>>>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?

Yes, or we could forget to do it. Adding it right now is another option that
Matthew has chosen and I don't see a strong reason to change it. Can you
measure a kernel build speedup thanks to removing the now redundant check?

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