Re: [PATCH v2] mm: swap: use fixed-size readahead during swapoff

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

Andrea Righi <andrea.righi@canonical.com> writes:

> On Wed, Apr 15, 2020 at 03:44:08PM +0800, Huang, Ying wrote:
>> Andrea Righi <andrea.righi@canonical.com> writes:
>> 
>> >  mm/swapfile.c | 4 +++-
>> >  1 file changed, 3 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/mm/swapfile.c b/mm/swapfile.c
>> > index 9fd47e6f7a86..cb9eb517178d 100644
>> > --- a/mm/swapfile.c
>> > +++ b/mm/swapfile.c
>> > @@ -1944,7 +1944,9 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>> >  		vmf.pmd = pmd;
>> >  		last_ra = atomic_read(&last_readahead_pages);
>> >  		atomic_set(&swapin_readahead_hits, last_ra);
>> 
>> You need to remove the above 2 lines firstly.
>
> Meh... too much enthusiasm, and I definitely need more coffee this
> morning. Here's the right patch applied:
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5871a2aa86a5..8b38441b66fa 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1940,7 +1940,9 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  		vmf.vma = vma;
>  		vmf.address = addr;
>  		vmf.pmd = pmd;
> -		page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, &vmf);
> +		page = lookup_swap_cache(entry, vma, addr);
> +		if (!page)
> +			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, &vmf);

The vmf assignment can be moved inside "if" block.  Otherwise the patch
looks good to me.

>  		if (!page) {
>  			if (*swap_map == 0 || *swap_map == SWAP_MAP_BAD)
>  				goto try_next;
>
> And following the right results:
>
> r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
> 	COUNT      EVENT
> 	1618       $retval = 1
> 	4960       $retval = 2
> 	41315      $retval = 4
> 	103521     $retval = 8
>
> swapoff time: 12.19s
>
> So, not as good as the fixed-size readahead, but it's definitely an
> improvement, considering that the swapoff time is ~22s without this
> patch applied.
>
> I think this change can be a simple and reasonable compromise.

Yes.  I think so too.

Best Regards,
Huang, Ying

> Thanks again and sorry for the noise,
> -Andrea