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

[Minchan Kim]

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 :/
> 
> On your second patch:
> 
> > @@ -1125,8 +1125,28 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		    !PageSwapCache(page)) {
> >  			if (!(sc->gfp_mask & __GFP_IO))
> >  				goto keep_locked;
> > -			if (!add_to_swap(page, page_list))
> > +swap_retry:
> > +			/*
> > +			 * Retry after split if we fail to allocate
> > +			 * swap space of a THP.
> > +			 */
> > +			if (!add_to_swap(page)) {
> > +				if (!PageTransHuge(page) ||
> > +				    split_huge_page_to_list(page, page_list))
> > +					goto activate_locked;
> > +				goto swap_retry;
> > +			}
> 
> This is definitely better.

Thanks.

> 
> However, I think it'd be cleaner without the label here:
> 
> 			if (!add_to_swap(page)) {
> 				if (!PageTransHuge(page))
> 					goto activate_locked;
> 				/* Split THP and swap individual base pages */
> 				if (split_huge_page_to_list(page, page_list))
> 					goto activate_locked;
> 				if (!add_to_swap(page))
> 					goto activate_locked;

Yes.

> 			}
> 
> > +			/*
> > +			 * Got swap space successfully. But unfortunately,
> > +			 * we don't support a THP page writeout so split it.
> > +			 */
> > +			if (PageTransHuge(page) &&
> > +				  split_huge_page_to_list(page, page_list)) {
> > +				delete_from_swap_cache(page);
> >  				goto activate_locked;
> > +			}
> 
> Pulling this out of add_to_swap() is an improvement for sure. Add an
> XXX: before that "we don't support THP writes" comment for good
> measure :)

Sure.

It could be a separate patch which makes add_to_swap clean via
removing page_list argument but I hope Huang take/fold it when he
resend it because it would be more important with THP swap.

Thanks.