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Date:   Thu, 27 Jan 2022 01:10:54 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Wei Yang <richard.weiyang@gmail.com>,
        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
Subject: Re: [PATCH] mm: remove offset check on page->compound_head and
 folio->lru
Message-ID: <20220127011054.zlqtydxbhi4ioj5d@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
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On Tue, Jan 25, 2022 at 11:11:40AM +0100, Vlastimil Babka wrote:
>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?
>

If we forget to do it, the compile would fail, right?

Put it here for a future reason is not persuasive.

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

-- 
Wei Yang
Help you, Help me