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Date:   Sun, 9 Jun 2019 14:53:35 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     ChenGang <cg.chen@huawei.com>
Cc:     akpm@linux-foundation.org, mhocko@suse.com, vbabka@suse.cz,
        osalvador@suse.de, pavel.tatashin@microsoft.com,
        mgorman@techsingularity.net, rppt@linux.ibm.com,
        richard.weiyang@gmail.com, alexander.h.duyck@linux.intel.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: align up min_free_kbytes to multipy of 4
Message-ID: <20190609145335.yzx4irt4mczmlvno@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
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On Sun, Jun 09, 2019 at 05:10:28PM +0800, ChenGang wrote:
>Usually the value of min_free_kbytes is multiply of 4,
>and in this case ,the right shift is ok.
>But if it's not, the right-shifting operation will lose the low 2 bits,

But PAGE_SHIFT is not always 12.

>and this cause kernel don't reserve enough memory.
>So it's necessary to align the value of min_free_kbytes to multiply of 4.
>For example, if min_free_kbytes is 64, then should keep 16 pages,
>but if min_free_kbytes is 65 or 66, then should keep 17 pages.
>
>Signed-off-by: ChenGang <cg.chen@huawei.com>
>---
> mm/page_alloc.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>index d66bc8a..1baeeba 100644
>--- a/mm/page_alloc.c
>+++ b/mm/page_alloc.c
>@@ -7611,7 +7611,8 @@ static void setup_per_zone_lowmem_reserve(void)
> 
> static void __setup_per_zone_wmarks(void)
> {
>-	unsigned long pages_min = min_free_kbytes >> (PAGE_SHIFT - 10);
>+	unsigned long pages_min =
>+		(PAGE_ALIGN(min_free_kbytes * 1024) / 1024) >> (PAGE_SHIFT - 10);

In my mind, pages_min is an estimated value. Do we need to be so precise?

> 	unsigned long lowmem_pages = 0;
> 	struct zone *zone;
> 	unsigned long flags;
>-- 
>1.8.5.6

-- 
Wei Yang
Help you, Help me