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Date:   Thu, 11 Aug 2022 16:07:39 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Kairui Song <kasong@tencent.com>
Cc:     Kairui Song <ryncsn@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] mm/util: reduce stack usage of folio_mapcount
Message-Id: <20220811160739.ea6b0f9c2912317c49287cda@linux-foundation.org>
In-Reply-To: <20220801173155.92008-1-ryncsn@gmail.com>
References: <20220801173155.92008-1-ryncsn@gmail.com>
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On Tue,  2 Aug 2022 01:31:55 +0800 Kairui Song <ryncsn@gmail.com> wrote:

> From: Kairui Song <kasong@tencent.com>
> 
> folio_entire_mapcount will call PageHeadHuge which is a function call,
> and blocks the compiler from recognizing this redundant load.

Did you mean folio_test_hugetlb() rather than folio_entire_mapcount()?


> After rearranging the code, stack usage is dropped from 32 to 24, and
> the function size is smaller (tested on GCC 12):
> 
> Before:
> Stack usage:
> mm/util.c:845:5:folio_mapcount  32      static
> Size:
> 0000000000000ea0 00000000000000c7 T folio_mapcount
> 
> After:
> Stack usage:
> mm/util.c:845:5:folio_mapcount  24      static
> Size:
> 0000000000000ea0 00000000000000b0 T folio_mapcount
> 
> ...
>
> @@ -850,10 +850,10 @@ int folio_mapcount(struct folio *folio)
>  		return atomic_read(&folio->_mapcount) + 1;
>  
>  	compound = folio_entire_mapcount(folio);
> -	nr = folio_nr_pages(folio);
>  	if (folio_test_hugetlb(folio))
>  		return compound;
>  	ret = compound;
> +	nr = folio_nr_pages(folio);
>  	for (i = 0; i < nr; i++)
>  		ret += atomic_read(&folio_page(folio, i)->_mapcount) + 1;
>  	/* File pages has compound_mapcount included in _mapcount */
> -- 
> 2.35.2