Re: [PATCH] mm: Add PageLayzyFree() helper functions for MADV_FREE

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  3 Mar 2020 11:37:38 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:

> From: Huang Ying <ying.huang@intel.com>
> 
> Now PageSwapBacked() is used as the helper function to check whether
> pages have been freed lazily via MADV_FREE.  This isn't very obvious.
> So Dave suggested to add PageLazyFree() family helper functions to
> improve the code readability.
> 
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -498,6 +498,31 @@ static __always_inline int PageKsm(struct page *page)
>  TESTPAGEFLAG_FALSE(Ksm)
>  #endif
>  
> +/*
> + * For pages freed lazily via MADV_FREE.  lazyfree pages are clean
> + * anonymous pages.  They have SwapBacked flag cleared to distinguish
> + * with normal anonymous pages
> + */
> +static __always_inline int PageLazyFree(struct page *page)
> +{
> +	page = compound_head(page);
> +	return PageAnon(page) && !PageSwapBacked(page);
> +}
> +
> +static __always_inline void SetPageLazyFree(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(PageTail(page), page);
> +	VM_BUG_ON_PAGE(!PageAnon(page), page);
> +	ClearPageSwapBacked(page);
> +}
> +
> +static __always_inline void ClearPageLazyFree(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(PageTail(page), page);
> +	VM_BUG_ON_PAGE(!PageAnon(page), page);
> +	SetPageSwapBacked(page);
> +}

These BUG_ONs aren't present in the current code and are
unchangelogged.

And they aren't free!

before:

q:/usr/src/25> size -t mm/rmap.o mm/swap.o mm/vmscan.o 
   text    data     bss     dec     hex filename
  41580    4211     192   45983    b39f mm/rmap.o
  50684    9259    1184   61127    eec7 mm/swap.o
 117728   46775     128  164631   28317 mm/vmscan.o
 209992   60245    1504  271741   4257d (TOTALS)

after:

   text    data     bss     dec     hex filename
  42035    4299     192   46526    b5be mm/rmap.o
  51091    9347    1184   61622    f0b6 mm/swap.o
 118132   46775     128  165035   284ab mm/vmscan.o
 211258   60421    1504  273183   42b1f (TOTALS)

ALso, if they actually trigger, Linus goes berserk.

If you have some special reason to add these assertions, please do it
as a (changelogged!) followup patch and I'll keep it as a linux-next only thing.