Re: [PATCH 8/9] swap_info: note SWAP_MAP_SHMEM

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 15 Oct 2009 01:57:28 +0100 (BST)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> While we're fiddling with the swap_map values, let's assign a particular
> value to shmem/tmpfs swap pages: their swap counts are never incremented,
> and it helps swapoff's try_to_unuse() a little if it can immediately
> distinguish those pages from process pages.
> 
> Since we've no use for SWAP_MAP_BAD | COUNT_CONTINUED,
> we might as well use that 0xbf value for SWAP_MAP_SHMEM.
> 
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

I welcome this!

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

> ---
> 
>  include/linux/swap.h |    6 +++++
>  mm/shmem.c           |   11 +++++++--
>  mm/swapfile.c        |   47 +++++++++++++++++++++++------------------
>  3 files changed, 42 insertions(+), 22 deletions(-)
> 
> --- si7/include/linux/swap.h	2009-10-14 21:26:57.000000000 +0100
> +++ si8/include/linux/swap.h	2009-10-14 21:27:07.000000000 +0100
> @@ -157,6 +157,7 @@ enum {
>  #define SWAP_HAS_CACHE	0x40	/* Flag page is cached, in first swap_map */
>  #define SWAP_CONT_MAX	0x7f	/* Max count, in each swap_map continuation */
>  #define COUNT_CONTINUED	0x80	/* See swap_map continuation for full count */
> +#define SWAP_MAP_SHMEM	0xbf	/* Owned by shmem/tmpfs, in first swap_map */
>  
>  /*
>   * The in-memory structure used to track swap areas.
> @@ -315,6 +316,7 @@ extern swp_entry_t get_swap_page(void);
>  extern swp_entry_t get_swap_page_of_type(int);
>  extern int valid_swaphandles(swp_entry_t, unsigned long *);
>  extern int add_swap_count_continuation(swp_entry_t, gfp_t);
> +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);
> @@ -393,6 +395,10 @@ static inline int add_swap_count_continu
>  	return 0;
>  }
>  
> +static inline void swap_shmem_alloc(swp_entry_t swp)
> +{
> +}
> +
>  static inline int swap_duplicate(swp_entry_t swp)
>  {
>  	return 0;
> --- si7/mm/shmem.c	2009-09-28 00:28:41.000000000 +0100
> +++ si8/mm/shmem.c	2009-10-14 21:27:07.000000000 +0100
> @@ -1017,7 +1017,14 @@ int shmem_unuse(swp_entry_t entry, struc
>  			goto out;
>  	}
>  	mutex_unlock(&shmem_swaplist_mutex);
> -out:	return found;	/* 0 or 1 or -ENOMEM */
> +	/*
> +	 * Can some race bring us here?  We've been holding page lock,
> +	 * so I think not; but would rather try again later than BUG()
> +	 */
> +	unlock_page(page);
> +	page_cache_release(page);
> +out:
> +	return (found < 0) ? found : 0;
>  }
>  
>  /*
> @@ -1080,7 +1087,7 @@ static int shmem_writepage(struct page *
>  		else
>  			inode = NULL;
>  		spin_unlock(&info->lock);
> -		swap_duplicate(swap);
> +		swap_shmem_alloc(swap);
>  		BUG_ON(page_mapped(page));
>  		page_cache_release(page);	/* pagecache ref */
>  		swap_writepage(page, wbc);
> --- si7/mm/swapfile.c	2009-10-14 21:26:57.000000000 +0100
> +++ si8/mm/swapfile.c	2009-10-14 21:27:07.000000000 +0100
> @@ -548,6 +548,12 @@ static unsigned char swap_entry_free(str
>  	if (usage == SWAP_HAS_CACHE) {
>  		VM_BUG_ON(!has_cache);
>  		has_cache = 0;
> +	} else if (count == SWAP_MAP_SHMEM) {
> +		/*
> +		 * Or we could insist on shmem.c using a special
> +		 * swap_shmem_free() and free_shmem_swap_and_cache()...
> +		 */
> +		count = 0;
>  	} else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
>  		if (count == COUNT_CONTINUED) {
>  			if (swap_count_continued(p, offset, count))
> @@ -1031,7 +1037,6 @@ static int try_to_unuse(unsigned int typ
>  	swp_entry_t entry;
>  	unsigned int i = 0;
>  	int retval = 0;
> -	int shmem;
>  
>  	/*
>  	 * When searching mms for an entry, a good strategy is to
> @@ -1107,17 +1112,18 @@ static int try_to_unuse(unsigned int typ
>  
>  		/*
>  		 * Remove all references to entry.
> -		 * Whenever we reach init_mm, there's no address space
> -		 * to search, but use it as a reminder to search shmem.
>  		 */
> -		shmem = 0;
>  		swcount = *swap_map;
> -		if (swap_count(swcount)) {
> -			if (start_mm == &init_mm)
> -				shmem = shmem_unuse(entry, page);
> -			else
> -				retval = unuse_mm(start_mm, entry, page);
> +		if (swap_count(swcount) == SWAP_MAP_SHMEM) {
> +			retval = shmem_unuse(entry, page);
> +			/* page has already been unlocked and released */
> +			if (retval < 0)
> +				break;
> +			continue;
>  		}
> +		if (swap_count(swcount) && start_mm != &init_mm)
> +			retval = unuse_mm(start_mm, entry, page);
> +
>  		if (swap_count(*swap_map)) {
>  			int set_start_mm = (*swap_map >= swcount);
>  			struct list_head *p = &start_mm->mmlist;
> @@ -1128,7 +1134,7 @@ static int try_to_unuse(unsigned int typ
>  			atomic_inc(&new_start_mm->mm_users);
>  			atomic_inc(&prev_mm->mm_users);
>  			spin_lock(&mmlist_lock);
> -			while (swap_count(*swap_map) && !retval && !shmem &&
> +			while (swap_count(*swap_map) && !retval &&
>  					(p = p->next) != &start_mm->mmlist) {
>  				mm = list_entry(p, struct mm_struct, mmlist);
>  				if (!atomic_inc_not_zero(&mm->mm_users))
> @@ -1142,10 +1148,9 @@ static int try_to_unuse(unsigned int typ
>  				swcount = *swap_map;
>  				if (!swap_count(swcount)) /* any usage ? */
>  					;
> -				else if (mm == &init_mm) {
> +				else if (mm == &init_mm)
>  					set_start_mm = 1;
> -					shmem = shmem_unuse(entry, page);
> -				} else
> +				else
>  					retval = unuse_mm(mm, entry, page);
>  
>  				if (set_start_mm &&
> @@ -1162,13 +1167,6 @@ static int try_to_unuse(unsigned int typ
>  			mmput(start_mm);
>  			start_mm = new_start_mm;
>  		}
> -		if (shmem) {
> -			/* page has already been unlocked and released */
> -			if (shmem > 0)
> -				continue;
> -			retval = shmem;
> -			break;
> -		}
>  		if (retval) {
>  			unlock_page(page);
>  			page_cache_release(page);
> @@ -2128,6 +2126,15 @@ bad_file:
>  }
>  
>  /*
> + * Help swapoff by noting that swap entry belongs to shmem/tmpfs
> + * (in which case its reference count is never incremented).
> + */
> +void swap_shmem_alloc(swp_entry_t entry)
> +{
> +	__swap_duplicate(entry, SWAP_MAP_SHMEM);
> +}
> +
> +/*
>   * increase reference count of swap entry by 1.
>   */
>  int swap_duplicate(swp_entry_t entry)
> 
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