Re: [PATCH -mm] mm, swap, frontswap: Fix THP swap if frontswap enabled

[Sergey Senozhatsky]

Hello,

On (02/06/18 01:02), Minchan Kim wrote:
[..]
> Can't we simple do like that if you want to make it simple and rely on someone
> who makes frontswap THP-aware later?
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 42fe5653814a..4bf1725407aa 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -934,7 +934,11 @@ int get_swap_pages(int n_goal, bool cluster, swp_entry_t swp_entries[])
>  
>         /* Only single cluster request supported */
>         WARN_ON_ONCE(n_goal > 1 && cluster);
> +#ifdef CONFIG_FRONTSWAP

Wouldn't #ifdef CONFIG_THP_SWAP be better? frontswap_enabled() is 'false'
on CONFIG_FRONTSWAP configs, should be compiled out anyway.

> +       /* Now, frontswap doesn't support THP page */
> +       if (frontswap_enabled() && cluster)
> +               return;
> +#endif
>         avail_pgs = atomic_long_read(&nr_swap_pages) / nr_pages;
>         if (avail_pgs <= 0)
>                 goto noswap;

Looks interesting. Technically, can be done earlier - in get_swap_page(),
can't it? get_swap_page() has the PageTransHuge(page) && CONFIG_THP_SWAP
condition checks. Can add frontswap dependency there. Something like

	if (PageTransHuge(page)) {
		if (IS_ENABLED(CONFIG_THP_SWAP))
+			if (!frontswap_enabled())
				get_swap_pages(1, true, &entry);
		return entry;
	}

	-ss