Re: [v3 PATCH 1/2] mm: swap: check if swap backing device is congested or not

[Tim Chen <tim.c.chen@linux.intel.com>]

On 12/21/18 1:40 PM, Yang Shi 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.
> 
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Tim Chen <tim.c.chen@intel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
> v3: Move inode deference under swap device type check per Tim Chen
> v2: Check the swap device type per Tim Chen
> 
>  mm/swap_state.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index fd2f21e..78d500e 100644
> --- 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;
> 

Acked-by: Tim Chen <tim.c.chen@linux.intel.com>