linux-kernel - Re: [v5 PATCH 1/2] mm: swap: check if swap backing device is congested or not

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Message-Id: <20190110152026.9e622df4884d4931c79b971d@linux-foundation.org>
Date:   Thu, 10 Jan 2019 15:20:26 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     ying.huang@...el.com, tim.c.chen@...el.com, minchan@...nel.org,
        daniel.m.jordan@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v5 PATCH 1/2] mm: swap: check if swap backing device is
 congested or not

On Fri,  4 Jan 2019 03:27:52 +0800 Yang Shi <yang.shi@...ux.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.
> 
> ...
>
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -538,11 +538,18 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  	bool do_poll = true, page_allocated;
>  	struct vm_area_struct *vma = vmf->vma;
>  	unsigned long addr = vmf->address;
> +	struct inode *inode = NULL;
>  
>  	mask = swapin_nr_pages(offset) - 1;
>  	if (!mask)
>  		goto skip;
>  
> +	if (si->flags & (SWP_BLKDEV | SWP_FS)) {
> +		inode = si->swap_file->f_mapping->host;
> +		if (inode_read_congested(inode))
> +			goto skip;
> +	}
> +
>  	do_poll = false;
>  	/* Read a page_cluster sized and aligned cluster around offset. */
>  	start_offset = offset & ~mask;

Neater:

--- a/mm/swap_state.c~mm-swap-check-if-swap-backing-device-is-congested-or-not-fix
+++ a/mm/swap_state.c
@@ -538,14 +538,13 @@ struct page *swap_cluster_readahead(swp_
 	bool do_poll = true, page_allocated;
 	struct vm_area_struct *vma = vmf->vma;
 	unsigned long addr = vmf->address;
-	struct inode *inode = NULL;
 
 	mask = swapin_nr_pages(offset) - 1;
 	if (!mask)
 		goto skip;
 
 	if (si->flags & (SWP_BLKDEV | SWP_FS)) {
-		inode = si->swap_file->f_mapping->host;
+		struct inode *inode = si->swap_file->f_mapping->host;
 		if (inode_read_congested(inode))
 			goto skip;
 	}
_