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Subject: Re: [PATCH 7/7] mm/khugepaged: try to free transhuge swapcache when
 possible
To:     Zach O'Keefe <zokeefe@google.com>
CC:     <akpm@linux-foundation.org>, <aarcange@redhat.com>,
        <willy@infradead.org>, <vbabka@suse.cz>, <dhowells@redhat.com>,
        <neilb@suse.de>, <apopple@nvidia.com>, <david@redhat.com>,
        <surenb@google.com>, <peterx@redhat.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
References: <20220611084731.55155-1-linmiaohe@huawei.com>
 <20220611084731.55155-8-linmiaohe@huawei.com> <YqoTU7SNM7d3MlNs@google.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <bec17d69-1002-25c3-d0f5-cc155ca65a67@huawei.com>
Date:   Thu, 16 Jun 2022 15:33:43 +0800
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On 2022/6/16 1:13, Zach O'Keefe wrote:
> On 11 Jun 16:47, Miaohe Lin wrote:
>> Transhuge swapcaches won't be freed in __collapse_huge_page_copy().
>> It's because release_pte_page() is not called for these pages and
>> thus free_page_and_swap_cache can't grab the page lock. These pages
>> won't be freed from swap cache even if we are the only user until
>> next time reclaim. It shouldn't hurt indeed, but we could try to
>> free these pages to save more memory for system.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  include/linux/swap.h | 5 +++++
>>  mm/khugepaged.c      | 1 +
>>  mm/swap.h            | 5 -----
>>  3 files changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 8672a7123ccd..ccb83b12b724 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -456,6 +456,7 @@ static inline unsigned long total_swapcache_pages(void)
>>  	return global_node_page_state(NR_SWAPCACHE);
>>  }
>>  
>> +extern void free_swap_cache(struct page *page);
>>  extern void free_page_and_swap_cache(struct page *);
>>  extern void free_pages_and_swap_cache(struct page **, int);
>>  /* linux/mm/swapfile.c */
>> @@ -540,6 +541,10 @@ static inline void put_swap_device(struct swap_info_struct *si)
>>  /* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
>>  #define free_swap_and_cache(e) is_pfn_swap_entry(e)
>>  
>> +static inline void free_swap_cache(struct page *page)
>> +{
>> +}
>> +
>>  static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
>>  {
>>  	return 0;
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index ee0a719c8be9..52109ad13f78 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -756,6 +756,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>>  	list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
>>  		list_del(&src_page->lru);
>>  		release_pte_page(src_page);
>> +		free_swap_cache(src_page);
>>  	}
>>  }
> 
> Aside: in __collapse_huge_page_isolate() (and also here) why can't we just check
> PageCompound(page) && page == compound_head(page) to only act on compound pages
> once? AFAIK this would alleviate this compound_pagelist business..
> 
> Anyways, as-is, free_page_and_swap_cache() won't be able to do
> try_to_free_swap(), since it can't grab page lock, put it will call put_page().
> I think (?) the last page ref might be dropped in release_pte_page(), so should
> free_swap_cache() come before it?

Thanks for catching this! If page is not in swapcache, the last page ref might be dropped.
So it's bad to call free_swap_cache() after it. Thanks!

> 
>>  
>> diff --git a/mm/swap.h b/mm/swap.h
>> index 0193797b0c92..863f6086c916 100644
>> --- a/mm/swap.h
>> +++ b/mm/swap.h
>> @@ -41,7 +41,6 @@ void __delete_from_swap_cache(struct page *page,
>>  void delete_from_swap_cache(struct page *page);
>>  void clear_shadow_from_swap_cache(int type, unsigned long begin,
>>  				  unsigned long end);
>> -void free_swap_cache(struct page *page);
>>  struct page *lookup_swap_cache(swp_entry_t entry,
>>  			       struct vm_area_struct *vma,
>>  			       unsigned long addr);
>> @@ -81,10 +80,6 @@ static inline struct address_space *swap_address_space(swp_entry_t entry)
>>  	return NULL;
>>  }
>>  
>> -static inline void free_swap_cache(struct page *page)
>> -{
>> -}
>> -
>>  static inline void show_swap_cache_info(void)
>>  {
>>  }
>> -- 
>> 2.23.0
>>
>>
> .
>