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Date:   Fri, 28 Dec 2018 16:42:46 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Yang Shi <yang.shi@linux.alibaba.com>
Cc:     ying.huang@intel.com, tim.c.chen@intel.com, minchan@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [v3 PATCH 1/2] mm: swap: check if swap backing device is
 congested or not
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On Sat, 22 Dec 2018 05:40:19 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:

> Swap readahead would read in a few pages regardless if the underlying
> device is busy or not.  It may incur long waiting time if the device is
> congested, and it may also exacerbate the congestion.
> 
> Use inode_read_congested() to check if the underlying device is busy or
> not like what file page readahead does.  Get inode from swap_info_struct.
> Although we can add inode information in swap_address_space
> (address_space->host), it may lead some unexpected side effect, i.e.
> it may break mapping_cap_account_dirty().  Using inode from
> swap_info_struct seems simple and good enough.
> 
> Just does the check in vma_cluster_readahead() since
> swap_vma_readahead() is just used for non-rotational device which
> much less likely has congestion than traditional HDD.
> 
> Although swap slots may be consecutive on swap partition, it still may be
> fragmented on swap file. This check would help to reduce excessive stall
> for such case.

Some words about the observed effects of the patch would be more than
appropriate!