Re: [PATCH 1/2] mm: Use owner_priv bit for PageSwapCache, valid when PageSwapBacked

[Hugh Dickins <hughd@google.com>]

On Sun, 25 Dec 2016, Nicholas Piggin wrote:

> A page is not added to the swap cache without being swap backed,
> so PageSwapBacked mappings can use PG_owner_priv_1 for PageSwapCache.
> 
> Acked-by: Hugh Dickins <hughd@google.com>

Yes, confirmed, Acked-by: Hugh Dickins <hughd@google.com>
I checked through your migrate and memory-failure additions,
and both look correct to me.  I still think that more should
be done for KPF_SWAPCACHE, to exclude the new false positives;
but as I said before, no urgency, that can be a later followup.

> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  include/linux/page-flags.h     | 24 ++++++++++++++++--------
>  include/trace/events/mmflags.h |  1 -
>  mm/memory-failure.c            |  4 +---
>  mm/migrate.c                   | 14 ++++++++------
>  4 files changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 74e4dda91238..a57c909a15e4 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -87,7 +87,6 @@ enum pageflags {
>  	PG_private_2,		/* If pagecache, has fs aux data */
>  	PG_writeback,		/* Page is under writeback */
>  	PG_head,		/* A head page */
> -	PG_swapcache,		/* Swap page: swp_entry_t in private */
>  	PG_mappedtodisk,	/* Has blocks allocated on-disk */
>  	PG_reclaim,		/* To be reclaimed asap */
>  	PG_swapbacked,		/* Page is backed by RAM/swap */
> @@ -110,6 +109,9 @@ enum pageflags {
>  	/* Filesystems */
>  	PG_checked = PG_owner_priv_1,
>  
> +	/* SwapBacked */
> +	PG_swapcache = PG_owner_priv_1,	/* Swap page: swp_entry_t in private */
> +
>  	/* Two page bits are conscripted by FS-Cache to maintain local caching
>  	 * state.  These bits are set on pages belonging to the netfs's inodes
>  	 * when those inodes are being locally cached.
> @@ -314,7 +316,13 @@ PAGEFLAG_FALSE(HighMem)
>  #endif
>  
>  #ifdef CONFIG_SWAP
> -PAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
> +static __always_inline int PageSwapCache(struct page *page)
> +{
> +	return PageSwapBacked(page) && test_bit(PG_swapcache, &page->flags);
> +
> +}
> +SETPAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
> +CLEARPAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
>  #else
>  PAGEFLAG_FALSE(SwapCache)
>  #endif
> @@ -701,12 +709,12 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
>   * Flags checked when a page is freed.  Pages being freed should not have
>   * these flags set.  It they are, there is a problem.
>   */
> -#define PAGE_FLAGS_CHECK_AT_FREE \
> -	(1UL << PG_lru	 | 1UL << PG_locked    | \
> -	 1UL << PG_private | 1UL << PG_private_2 | \
> -	 1UL << PG_writeback | 1UL << PG_reserved | \
> -	 1UL << PG_slab	 | 1UL << PG_swapcache | 1UL << PG_active | \
> -	 1UL << PG_unevictable | __PG_MLOCKED)
> +#define PAGE_FLAGS_CHECK_AT_FREE				\
> +	(1UL << PG_lru		| 1UL << PG_locked	|	\
> +	 1UL << PG_private	| 1UL << PG_private_2	|	\
> +	 1UL << PG_writeback	| 1UL << PG_reserved	|	\
> +	 1UL << PG_slab		| 1UL << PG_active 	|	\
> +	 1UL << PG_unevictable	| __PG_MLOCKED)
>  
>  /*
>   * Flags checked when a page is prepped for return by the page allocator.
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 5a81ab48a2fb..30c2adbdebe8 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -95,7 +95,6 @@
>  	{1UL << PG_private_2,		"private_2"	},		\
>  	{1UL << PG_writeback,		"writeback"	},		\
>  	{1UL << PG_head,		"head"		},		\
> -	{1UL << PG_swapcache,		"swapcache"	},		\
>  	{1UL << PG_mappedtodisk,	"mappedtodisk"	},		\
>  	{1UL << PG_reclaim,		"reclaim"	},		\
>  	{1UL << PG_swapbacked,		"swapbacked"	},		\
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 19e796d36a62..f283c7e0a2a3 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -764,12 +764,11 @@ static int me_huge_page(struct page *p, unsigned long pfn)
>   */
>  
>  #define dirty		(1UL << PG_dirty)
> -#define sc		(1UL << PG_swapcache)
> +#define sc		((1UL << PG_swapcache) | (1UL << PG_swapbacked))
>  #define unevict		(1UL << PG_unevictable)
>  #define mlock		(1UL << PG_mlocked)
>  #define writeback	(1UL << PG_writeback)
>  #define lru		(1UL << PG_lru)
> -#define swapbacked	(1UL << PG_swapbacked)
>  #define head		(1UL << PG_head)
>  #define slab		(1UL << PG_slab)
>  #define reserved	(1UL << PG_reserved)
> @@ -819,7 +818,6 @@ static struct page_state {
>  #undef mlock
>  #undef writeback
>  #undef lru
> -#undef swapbacked
>  #undef head
>  #undef slab
>  #undef reserved
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 0ed24b1fa77b..87f4d0f81819 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -466,13 +466,15 @@ int migrate_page_move_mapping(struct address_space *mapping,
>  	 */
>  	newpage->index = page->index;
>  	newpage->mapping = page->mapping;
> -	if (PageSwapBacked(page))
> -		__SetPageSwapBacked(newpage);
> -
>  	get_page(newpage);	/* add cache reference */
> -	if (PageSwapCache(page)) {
> -		SetPageSwapCache(newpage);
> -		set_page_private(newpage, page_private(page));
> +	if (PageSwapBacked(page)) {
> +		__SetPageSwapBacked(newpage);
> +		if (PageSwapCache(page)) {
> +			SetPageSwapCache(newpage);
> +			set_page_private(newpage, page_private(page));
> +		}
> +	} else {
> +		VM_BUG_ON_PAGE(PageSwapCache(page), page);
>  	}
>  
>  	/* Move dirty while page refs frozen and newpage not yet exposed */
> -- 
> 2.11.0