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Date:   Tue, 4 Jul 2023 11:06:59 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Andrew Yang <andrew.yang@mediatek.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        <wsd_upstream@mediatek.com>, <casper.li@mediatek.com>,
        <linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-mediatek@lists.infradead.org>, linux-mm@kvack.org
Subject: Re: [PATCH] fs: drop_caches: draining pages before dropping caches
Message-Id: <20230704110659.3e1de8001f9208e7278352e8@linux-foundation.org>
In-Reply-To: <20230630092203.16080-1-andrew.yang@mediatek.com>
References: <20230630092203.16080-1-andrew.yang@mediatek.com>
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On Fri, 30 Jun 2023 17:22:02 +0800 Andrew Yang <andrew.yang@mediatek.com> wrote:

> We expect a file page access after dropping caches should be a major
> fault, but sometimes it's still a minor fault. That's because a file
> page can't be dropped if it's in a per-cpu pagevec. Draining all pages
> from per-cpu pagevec to lru list before trying to drop caches.
> 
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -10,6 +10,7 @@
>  #include <linux/writeback.h>
>  #include <linux/sysctl.h>
>  #include <linux/gfp.h>
> +#include <linux/swap.h>
>  #include "internal.h"
>  
>  /* A global variable is a bit ugly, but it keeps the code simple */
> @@ -59,6 +60,7 @@ int drop_caches_sysctl_handler(struct ctl_table *table, int write,
>  		static int stfu;
>  
>  		if (sysctl_drop_caches & 1) {
> +			lru_add_drain_all();
>  			iterate_supers(drop_pagecache_sb, NULL);
>  			count_vm_event(DROP_PAGECACHE);
>  		}

um, yes.  I'm suprised that this oversight has survived 20+ years.