linux-kernel - Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru

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Date:   Wed, 9 Sep 2020 02:01:18 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        tj@...nel.org, hughd@...gle.com, khlebnikov@...dex-team.ru,
        daniel.m.jordan@...cle.com, hannes@...xchg.org, lkp@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, shakeelb@...gle.com,
        iamjoonsoo.kim@....com, richard.weiyang@...il.com,
        kirill@...temov.name, alexander.duyck@...il.com,
        rong.a.chen@...el.com, mhocko@...e.com, vdavydov.dev@...il.com,
        shy828301@...il.com,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception
 path for isolate_lru_pages

On Mon, Aug 24, 2020 at 08:55:04PM +0800, Alex Shi wrote:
> +++ b/mm/vmscan.c
> @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  
>  			if (!TestClearPageLRU(page)) {
>  				/*
> -				 * This page may in other isolation path,
> -				 * but we still hold lru_lock.
> +				 * This page is being isolated in another
> +				 * thread, but we still hold lru_lock. The
> +				 * other thread must be holding a reference
> +				 * to the page so this should never hit a
> +				 * reference count of 0.
>  				 */
> -				put_page(page);
> +				WARN_ON(put_page_testzero(page));
>  				goto busy;

I read Hugh's review and that led me to take a look at this.  We don't
do it like this.  Use the same pattern as elsewhere in mm:

        page_ref_sub(page, nr);
        VM_BUG_ON_PAGE(page_count(page) <= 0, page);