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

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Date:   Thu, 11 May 2017 08:50:01 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        "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

Minchan Kim <minchan@...nel.org> writes:

> 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.

Sure.  I will take this patch as one patch of the THP swap series.
Because the first patch of the THP swap series is a little big, I don't
think it is a good idea to fold this patch into it.  Could you update
the patch according to Johannes' comments and resend it?

Best Regards,
Huang, Ying