linux-kernel - Re: [PATCH v2] mm/page_alloc: fix counting of free pages after take off from buddy

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Date:   Tue, 25 May 2021 08:32:28 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Ding Hui <dinghui@...gfor.com.cn>
CC:     "david@...hat.com" <david@...hat.com>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm/page_alloc: fix counting of free pages after take
 off from buddy

On Sat, May 08, 2021 at 11:55:33AM +0800, Ding Hui wrote:
> Recently we found there is a lot MemFree left in /proc/meminfo after
> do a lot of pages soft offline.
> 
> I think it's incorrect since NR_FREE_PAGES should not contain HWPoison pages.
> For offline free pages, after a successful call take_page_off_buddy(), the
> page is no longer belong to buddy allocator, and will not be used any more,
> but we missed accounting NR_FREE_PAGES in this situation.
> 
> Do update like rmqueue() does.
> 
> Signed-off-by: Ding Hui <dinghui@...gfor.com.cn>
> ---
> V2:
> use __mod_zone_freepage_state instead of __mod_zone_page_state
> 
>  mm/page_alloc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cfc72873961d..e124a615303b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8947,6 +8947,7 @@ bool take_page_off_buddy(struct page *page)
>  			del_page_from_free_list(page_head, zone, page_order);
>  			break_down_buddy_pages(zone, page_head, page, 0,
>  						page_order, migratetype);
> +			__mod_zone_freepage_state(zone, -1, migratetype);

Page offline code (see set_migratetype_isolate()) seems to handle
NR_FREE_PAGES counter in its own way, so I think that it's more correct to
call __mod_zone_freepage_state() only when is_migrate_isolate(migratetype))
is false.

Otherwise, the patch looks good to me.

Thanks,
Naoya Horiguchi

>  			ret = true;
>  			break;
>  		}
> -- 
> 2.17.1
>