linux-kernel - Re: [PATCH v3] mm/page_alloc: fix counting of free pages after take off from buddy

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Message-ID: <20210527003436.GA3543069@hori.linux.bs1.fc.nec.co.jp>
Date:   Thu, 27 May 2021 00:34:36 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Ding Hui <dinghui@...gfor.com.cn>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "david@...hat.com" <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v3] mm/page_alloc: fix counting of free pages after take
 off from buddy

On Wed, May 26, 2021 at 03:52:47PM +0800, Ding Hui wrote:
> Recently we found that there is a lot MemFree left in /proc/meminfo
> after do a lot of pages soft offline, it's not quite correct.
> 
> Before Oscar rework soft offline for free pages [1], if we soft
> offline free pages, these pages are left in buddy with HWPoison
> flag, and NR_FREE_PAGES is not updated immediately. So the difference
> between NR_FREE_PAGES and real number of available free pages is
> also even big at the beginning.
> 
> However, with the workload running, when we catch HWPoison page in
> any alloc functions subsequently, we will remove it from buddy,
> meanwhile update the NR_FREE_PAGES and try again, so the NR_FREE_PAGES
> will get more and more closer to the real number of available free pages.
> (regardless of unpoison_memory())
> 
> Now, 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,
> and there is no chance to be updated later.
> 
> Do update in take_page_off_buddy() like rmqueue() does, but avoid
> double counting if some one already set_migratetype_isolate() on the
> page.
> 
> [1]: commit 06be6ff3d2ec ("mm,hwpoison: rework soft offline for free pages")
> 
> Suggested-by: Naoya Horiguchi <naoya.horiguchi@....com>
> Signed-off-by: Ding Hui <dinghui@...gfor.com.cn>

Thank you very much.

Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>

As for unpoison_memory(), I'm writing patches to fix unpoison (maybe takes a
few weeks to be posted) and that will add a reverse operation of
take_page_off_buddy() which simply calls __free_one_page(), so NR_FREE_PAGES
counter will also be handled correctly with the patches.