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

["Huang\, Ying" <ying.huang@intel.com>]

Hi, Andrew,

Andrew Morton <akpm@linux-foundation.org> writes:

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

In this patch, if the system is swapping to both a SSD and to a spinning
disk, we'll treat SSD as spinning disk.  That is, to use original swap
readahead algorithm instead of new proposed VMA based swap readahead
algorithm.

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

It's hard for VMA based swap readahead algorithm.  With that algorithm,
the PTEs near the fault address will be checked, some of them may come
from SSD and the others come from spinning disk, it is hard to choose
which algorithm to use for this situation.

So I choose a simple solution to use original swap readahead algorithm
if there is one spinning disk is used as swap, and hope most people
will not use both the spinning disk and SSD as swap at the same time.

>>  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.

There is such test in sys_swapon()

	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
                ...
        } else
                atomic_inc(&nr_rotate_swap);



I use it in swapoff to try to make counting symmetrical.  Do I
misunderstand the code?

Best Regards,
Huang, Ying

>
>>  	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

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