linux-kernel - Re: [PATCH -mm -v10 1/3] mm, THP, swap: Delay splitting THP during swap out

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Date:   Fri, 12 May 2017 10:34:16 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Ebru Akagunduz <ebru.akagunduz@...il.com>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Rik van Riel <riel@...hat.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH -mm -v10 1/3] mm, THP, swap: Delay splitting THP during
 swap out

Hi Hannes,

On Thu, May 11, 2017 at 06:40:58AM -0400, Johannes Weiner wrote:
> On Thu, May 11, 2017 at 10:22:13AM +0900, Minchan Kim wrote:
> > On Thu, May 11, 2017 at 08:25:56AM +0900, Minchan Kim wrote:
> > > On Wed, May 10, 2017 at 09:56:54AM -0400, Johannes Weiner wrote:
> > > > Hi Michan,
> > > > 
> > > > On Tue, May 02, 2017 at 08:53:32AM +0900, Minchan Kim wrote:
> > > > > @@ -1144,7 +1144,7 @@ void swap_free(swp_entry_t entry)
> > > > >  /*
> > > > >   * Called after dropping swapcache to decrease refcnt to swap entries.
> > > > >   */
> > > > > -void swapcache_free(swp_entry_t entry)
> > > > > +void __swapcache_free(swp_entry_t entry)
> > > > >  {
> > > > >  	struct swap_info_struct *p;
> > > > >  
> > > > > @@ -1156,7 +1156,7 @@ void swapcache_free(swp_entry_t entry)
> > > > >  }
> > > > >  
> > > > >  #ifdef CONFIG_THP_SWAP
> > > > > -void swapcache_free_cluster(swp_entry_t entry)
> > > > > +void __swapcache_free_cluster(swp_entry_t entry)
> > > > >  {
> > > > >  	unsigned long offset = swp_offset(entry);
> > > > >  	unsigned long idx = offset / SWAPFILE_CLUSTER;
> > > > > @@ -1182,6 +1182,14 @@ void swapcache_free_cluster(swp_entry_t entry)
> > > > >  }
> > > > >  #endif /* CONFIG_THP_SWAP */
> > > > >  
> > > > > +void swapcache_free(struct page *page, swp_entry_t entry)
> > > > > +{
> > > > > +	if (!PageTransHuge(page))
> > > > > +		__swapcache_free(entry);
> > > > > +	else
> > > > > +		__swapcache_free_cluster(entry);
> > > > > +}
> > > > 
> > > > I don't think this is cleaner :/
> > 
> > Let's see a example add_to_swap. Without it, it looks like that.
> > 
> > int add_to_swap(struct page *page)
> > {
> >         entry = get_swap_page(page);
> >         ..
> >         ..
> > fail:
> >         if (PageTransHuge(page))
> >                 swapcache_free_cluster(entry);
> >         else
> >                 swapcache_free(entry);
> > }
> > 
> > It doesn't looks good to me because get_swap_page hides
> > where entry allocation is from cluster or slot but when
> > we free the entry allocated, we should be aware of the
> > internal and call right function. :(
> 
> This could be nicer indeed. I just don't like the underscore versions
> much, but symmetry with get_swap_page() would be nice.
> 
> How about put_swap_page()? :)

Good idea. It's the best one I can do now.
Actually, get_swap_page is awkward to me. Maybe it would be nicer to
rename it with get_swap_[slot|entry] but, I will postpone it if someone
would be on same page with me in future.

> 
> That can call the appropriate swapcache_free function then.

Yub.
Thanks for the review!