Re: [PATCH -mm -v6 4/9] mm, THP, swap: Add get_huge_swap_page()

[Tim Chen]

On Wed, 2017-03-08 at 15:26 +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> A variation of get_swap_page(), get_huge_swap_page(), is added to
> allocate a swap cluster (HPAGE_PMD_NR swap slots) based on the swap
> cluster allocation function.ÂÂA fair simple algorithm is used, that is,
> only the first swap device in priority list will be tried to allocate
> the swap cluster.ÂÂThe function will fail if the trying is not
> successful, and the caller will fallback to allocate a single swap slot
> instead.ÂÂThis works good enough for normal cases.
> 
> This will be used for the THP (Transparent Huge Page) swap support.
> Where get_huge_swap_page() will be used to allocate one swap cluster for
> each THP swapped out.
> 
> Because of the algorithm adopted, if the difference of the number of the
> free swap clusters among multiple swap devices is significant, it is
> possible that some THPs are split earlier than necessary.ÂÂFor example,
> this could be caused by big size difference among multiple swap devices.
> 
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> ---
> Âinclude/linux/swap.h | 19 ++++++++++++++++++-
> Âmm/swap_slots.cÂÂÂÂÂÂ|ÂÂ5 +++--
> Âmm/swapfile.cÂÂÂÂÂÂÂÂ| 16 ++++++++++++----
> Â3 files changed, 33 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 278e1349a424..e3a7609a8989 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -388,7 +388,7 @@ static inline long get_nr_swap_pages(void)
> Âextern void si_swapinfo(struct sysinfo *);
> Âextern swp_entry_t get_swap_page(void);
> Âextern swp_entry_t get_swap_page_of_type(int);
> -extern int get_swap_pages(int n, swp_entry_t swp_entries[]);
> +extern int get_swap_pages(int n, swp_entry_t swp_entries[], bool huge);
> Âextern int add_swap_count_continuation(swp_entry_t, gfp_t);
> Âextern void swap_shmem_alloc(swp_entry_t);
> Âextern int swap_duplicate(swp_entry_t);
> @@ -527,6 +527,23 @@ static inline swp_entry_t get_swap_page(void)
> Â
> Â#endif /* CONFIG_SWAP */
> Â
> +#ifdef CONFIG_THP_SWAP_CLUSTER
> +static inline swp_entry_t get_huge_swap_page(void)
> +{
> +	swp_entry_t entry;
> +
> +	if (get_swap_pages(1, &entry, true))
> +		return entry;
> +	else
> +		return (swp_entry_t) {0};
> +}
> +#else
> +static inline swp_entry_t get_huge_swap_page(void)
> +{
> +	return (swp_entry_t) {0};
> +}
> +#endif
> +
> Â#ifdef CONFIG_MEMCG
> Âstatic inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> Â{
> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> index 9b5bc86f96ad..075bb39e03c5 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -258,7 +258,8 @@ static int refill_swap_slots_cache(struct swap_slots_cache *cache)
> Â
> Â	cache->cur = 0;
> Â	if (swap_slot_cache_active)
> -		cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE, cache->slots);
> +		cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE, cache->slots,
> +					ÂÂÂfalse);
> Â
> Â	return cache->nr;
> Â}
> @@ -334,7 +335,7 @@ swp_entry_t get_swap_page(void)
> Â			return entry;
> Â	}
> Â
> -	get_swap_pages(1, &entry);
> +	get_swap_pages(1, &entry, false);
> Â
> Â	return entry;
> Â}
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 91876c33114b..7241c937e52b 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -904,11 +904,12 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,
> Â
> Â}
> Â
> -int get_swap_pages(int n_goal, swp_entry_t swp_entries[])


> +int get_swap_pages(int n_goal, swp_entry_t swp_entries[], bool huge)
> Â{
> Â	struct swap_info_struct *si, *next;
> Â	long avail_pgs;
> Â	int n_ret = 0;
> +	int nr_pages = huge_cluster_nr_entries(huge);
> Â
> Â	avail_pgs = atomic_long_read(&nr_swap_pages);
> Â	if (avail_pgs <= 0)
> @@ -920,6 +921,10 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[])
> Â	if (n_goal > avail_pgs)
> Â		n_goal = avail_pgs;
> Â
> +	n_goal *= nr_pages;

I think if (n_goal > 1) when huge is true,Â
n_goal should be set to huge_cluster_nr_entries(huge) here
or we could have an invalid check below. We probably
should add a comment to get_swap_pages on how we treat
n_goal when huge is true. ÂMaybe say we will always treat
n_goal as SWAPFILE_CLUSTER when huge is true.Â

> +	if (avail_pgs < n_goal)
> +		goto noswap;
> +
> Â	atomic_long_sub(n_goal, &nr_swap_pages);
> Â
> Â	spin_lock(&swap_avail_lock);
> @@ -946,10 +951,13 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[])
> Â			spin_unlock(&si->lock);
> Â			goto nextsi;
> Â		}
> -		n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> -					ÂÂÂÂn_goal, swp_entries);
> +		if (likely(nr_pages == 1))

if (likely(!huge)) is probably more readable

> +			n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> +						ÂÂÂÂn_goal, swp_entries);
> +		else
> +			n_ret = swap_alloc_huge_cluster(si, swp_entries);
> Â		spin_unlock(&si->lock);

Thanks.

Tim