Re: [PATCH -v3 07/10] mm, THP, swap: Support to add/delete THP to/from swap cache

[Anshuman Khandual]

On 09/07/2016 10:16 PM, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> With this patch, a THP (Transparent Huge Page) can be added/deleted
> to/from the swap cache as a set of sub-pages (512 on x86_64).
> 
> This will be used for the THP (Transparent Huge Page) swap support.
> Where one THP may be added/delted to/from the swap cache.  This will
> batch the swap cache operations to reduce the lock acquire/release times
> for the THP swap too.
> 
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> ---
>  include/linux/page-flags.h |  2 +-
>  mm/swap_state.c            | 57 +++++++++++++++++++++++++++++++---------------
>  2 files changed, 40 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 74e4dda..f5bcbea 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -314,7 +314,7 @@ PAGEFLAG_FALSE(HighMem)
>  #endif
>  
>  #ifdef CONFIG_SWAP
> -PAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
> +PAGEFLAG(SwapCache, swapcache, PF_NO_TAIL)

What is the reason for this change ? The commit message does not seem
to explain.