linux-kernel - Re: [RFC PATCH] mm: don't reclaim inodes with many attached pages

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Date:   Wed, 24 Oct 2018 15:18:53 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC PATCH] mm: don't reclaim inodes with many attached pages

On Tue, 23 Oct 2018 16:43:29 +0000 Roman Gushchin <guro@...com> wrote:

> Spock reported that the commit 172b06c32b94 ("mm: slowly shrink slabs
> with a relatively small number of objects") leads to a regression on
> his setup: periodically the majority of the pagecache is evicted
> without an obvious reason, while before the change the amount of free
> memory was balancing around the watermark.
> 
> The reason behind is that the mentioned above change created some
> minimal background pressure on the inode cache. The problem is that
> if an inode is considered to be reclaimed, all belonging pagecache
> page are stripped, no matter how many of them are there. So, if a huge
> multi-gigabyte file is cached in the memory, and the goal is to
> reclaim only few slab objects (unused inodes), we still can eventually
> evict all gigabytes of the pagecache at once.
> 
> The workload described by Spock has few large non-mapped files in the
> pagecache, so it's especially noticeable.
> 
> To solve the problem let's postpone the reclaim of inodes, which have
> more than 1 attached page. Let's wait until the pagecache pages will
> be evicted naturally by scanning the corresponding LRU lists, and only
> then reclaim the inode structure.
> 
> ...
>
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -730,8 +730,11 @@ static enum lru_status inode_lru_isolate(struct list_head *item,
>  		return LRU_REMOVED;
>  	}
>  
> -	/* recently referenced inodes get one more pass */
> -	if (inode->i_state & I_REFERENCED) {
> +	/*
> +	 * Recently referenced inodes and inodes with many attached pages
> +	 * get one more pass.
> +	 */
> +	if (inode->i_state & I_REFERENCED || inode->i_data.nrpages > 1) {
>  		inode->i_state &= ~I_REFERENCED;
>  		spin_unlock(&inode->i_lock);
>  		return LRU_ROTATE;

hm, why "1"?

I guess one could argue that this will encompass long symlinks, but I
just made that up to make "1" appear more justifiable ;)