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

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Date:   Sat, 29 Jan 2022 00:47:39 +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 Thu, Jan 27, 2022 at 04:42:10PM +0100, Vlastimil Babka wrote:
>On 1/27/22 02:10, Wei Yang wrote:
>> 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:
>>>>>
>>>>>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?
>
>No, FOLIO_MATCH is like a build-time assert. It can only fail if the assert
>is there, and the condition evaluates to false.

Currently we have this check

  FOLIO_MATCH(lru, lru)

Which checks page->lru and folio->lru.

As you mentioned page->lru would be gone. So this check would fail at compile?


-- 
Wei Yang
Help you, Help me