linux-kernel - Re: [PATCH -mm -v6 5/9] mm, THP, swap: Support to clear SWAP_HAS

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Date:   Wed, 15 Mar 2017 08:54:44 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
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>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Shaohua Li" <shli@...nel.org>, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH -mm -v6 5/9] mm, THP, swap: Support to clear SWAP_HAS_CACHE for huge page

Tim Chen <tim.c.chen@...ux.intel.com> writes:

> On Wed, 2017-03-08 at 15:26 +0800, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@...el.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@...hat.com>
>> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>> Cc: Hugh Dickins <hughd@...gle.com>
>> Cc: Shaohua Li <shli@...nel.org>
>> Cc: Minchan Kim <minchan@...nel.org>
>> Cc: Rik van Riel <riel@...hat.com>
>> Signed-off-by: "Huang, Ying" <ying.huang@...el.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.

OK.  I will change this.

Best Regards,
Huang, Ying