linux-kernel - Re: [PATCH -mm -v3 6/6] mm, swap: Don't use VMA based swap readahead if HDD is used as swap

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Date:   Tue, 25 Jul 2017 13:50:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>, Shaohua Li <shli@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH -mm -v3 6/6] mm, swap: Don't use VMA based swap
 readahead if HDD is used as swap

On Tue, 25 Jul 2017 09:51:51 +0800 "Huang, Ying" <ying.huang@...el.com> wrote:

> From: Huang Ying <ying.huang@...el.com>
> 
> VMA based swap readahead will readahead the virtual pages that is
> continuous in the virtual address space.  While the original swap
> readahead will readahead the swap slots that is continuous in the swap
> device.  Although VMA based swap readahead is more correct for the
> swap slots to be readahead, it will trigger more small random
> readings, which may cause the performance of HDD (hard disk) to
> degrade heavily, and may finally exceed the benefit.
> 
> To avoid the issue, in this patch, if the HDD is used as swap, the VMA
> based swap readahead will be disabled, and the original swap readahead
> will be used instead.
>
> ...
> 
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -399,16 +399,17 @@ extern struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
>  					   struct vm_fault *vmf,
>  					   struct vma_swap_readahead *swap_ra);
>  
> -static inline bool swap_use_vma_readahead(void)
> -{
> -	return READ_ONCE(swap_vma_readahead);
> -}
> -
>  /* linux/mm/swapfile.c */
>  extern atomic_long_t nr_swap_pages;
>  extern long total_swap_pages;
> +extern atomic_t nr_rotate_swap;

This is rather ugly.  If the system is swapping to both an SSD and to a
spinning disk, we'll treat the spinning disk as SSD.

Surely this decision can be made in a per-device fashion?

>  extern bool has_usable_swap(void);
>  
> +static inline bool swap_use_vma_readahead(void)
> +{
> +	return READ_ONCE(swap_vma_readahead) && !atomic_read(&nr_rotate_swap);
> +}
> +
>  /* Swap 50% full? Release swapcache more aggressively.. */
>  static inline bool vm_swap_full(void)
>  {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 6ba4aab2db0b..2685b9951cc1 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -96,6 +96,8 @@ static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
>  /* Activity counter to indicate that a swapon or swapoff has occurred */
>  static atomic_t proc_poll_event = ATOMIC_INIT(0);
>  
> +atomic_t nr_rotate_swap = ATOMIC_INIT(0);
> +
>  static inline unsigned char swap_count(unsigned char ent)
>  {
>  	return ent & ~SWAP_HAS_CACHE;	/* may include SWAP_HAS_CONT flag */
> @@ -2387,6 +2389,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	if (p->flags & SWP_CONTINUED)
>  		free_swap_count_continuations(p);
>  
> +	if (!p->bdev || !blk_queue_nonrot(bdev_get_queue(p->bdev)))
> +		atomic_dec(&nr_rotate_swap);

What's that p->bdev test for?  It's not symmetrical with the
sys_swapon() change and one wonders if the counter can get out of sync.


>  	mutex_lock(&swapon_mutex);
>  	spin_lock(&swap_lock);
>  	spin_lock(&p->lock);
> @@ -2963,7 +2968,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  			cluster = per_cpu_ptr(p->percpu_cluster, cpu);
>  			cluster_set_null(&cluster->index);
>  		}
> -	}
> +	} else
> +		atomic_inc(&nr_rotate_swap);
>  
>  	error = swap_cgroup_swapon(p->type, maxpages);
>  	if (error)
> -- 
> 2.13.2