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

Andrea Righi
2020-04-15T09:20:16Z

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);
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.

Thanks again and sorry for the noise,
-Andrea