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

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Date:   Wed, 10 May 2017 09:56:54 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.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 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.

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;
			}

> +			/*
> +			 * 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 :)