Re: [PATCH -mm -v6 5/9] mm, THP, swap: Support to clear SWAP_HAS_CACHE for huge page

[Tim Chen <tim.c.chen@linux.intel.com>]

On Wed, 2017-03-08 at 15:26 +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.com>
> 
> __swapcache_free() is added to support to clear the SWAP_HAS_CACHE flag
> for the huge page.  This will free the specified swap cluster now.
> Because now this function will be called only in the error path to free
> the swap cluster just allocated.  So the corresponding swap_map[i] ==
> SWAP_HAS_CACHE, that is, the swap count is 0.  This makes the
> implementation simpler than that of the ordinary swap entry.
> 
> This will be used for delaying splitting THP (Transparent Huge Page)
> during swapping out.  Where for one THP to swap out, we will allocate a
> swap cluster, add the THP into the swap cache, then split the THP.  If
> anything fails after allocating the swap cluster and before splitting
> the THP successfully, the swapcache_free_trans_huge() will be used to
> free the swap space allocated.
> 
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Shaohua Li <shli@kernel.org>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Rik van Riel <riel@redhat.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> ---
>  include/linux/swap.h |  9 +++++++--
>  mm/swapfile.c        | 34 ++++++++++++++++++++++++++++++++--
>  2 files changed, 39 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index e3a7609a8989..2f2a6c0363aa 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -394,7 +394,7 @@ extern void swap_shmem_alloc(swp_entry_t);
>  extern int swap_duplicate(swp_entry_t);
>  extern int swapcache_prepare(swp_entry_t);
>  extern void swap_free(swp_entry_t);
> -extern void swapcache_free(swp_entry_t);
> +extern void __swapcache_free(swp_entry_t entry, bool huge);
>  extern void swapcache_free_entries(swp_entry_t *entries, int n);
>  extern int free_swap_and_cache(swp_entry_t);
>  extern int swap_type_of(dev_t, sector_t, struct block_device **);
> @@ -456,7 +456,7 @@ static inline void swap_free(swp_entry_t swp)
>  {
>  }
>  
> -static inline void swapcache_free(swp_entry_t swp)
> +static inline void __swapcache_free(swp_entry_t swp, bool huge)
>  {
>  }
>  
> @@ -544,6 +544,11 @@ static inline swp_entry_t get_huge_swap_page(void)
>  }
>  #endif
>  
> +static inline void swapcache_free(swp_entry_t entry)
> +{
> +	__swapcache_free(entry, false);
> +}
> +
>  #ifdef CONFIG_MEMCG
>  static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
>  {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 7241c937e52b..6019f94afbaf 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -855,6 +855,29 @@ static void swap_free_huge_cluster(struct swap_info_struct *si,
>  	_swap_entry_free(si, offset, true);
>  }
>  
> +static void swapcache_free_trans_huge(struct swap_info_struct *si,
> +				      swp_entry_t entry)
> +{
> +	unsigned long offset = swp_offset(entry);
> +	unsigned long idx = offset / SWAPFILE_CLUSTER;
> +	struct swap_cluster_info *ci;
> +	unsigned char *map;
> +	unsigned int i;
> +
> +	spin_lock(&si->lock);
> +	ci = lock_cluster(si, offset);
> +	map = si->swap_map + offset;
> +	for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> +		VM_BUG_ON(map[i] != SWAP_HAS_CACHE);
> +		map[i] &= ~SWAP_HAS_CACHE;

Nitpicking a bit:
map[i] = 0  is more readable if map[i] == SWAP_HAS_CACHE here.


Thanks.

Tim