Re: [patch] mm: release swap slots for actively used pages

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

On Wed, 27 May 2009 03:47:39 +0200
Johannes Weiner <hannes@cmpxchg.org> wrote:

> For anonymous pages activated by the reclaim scan or faulted from an
> evicted page table entry we should always try to free up swap space.
> 
> Both events indicate that the page is in active use and a possible
> change in the working set.  Thus removing the slot association from
> the page increases the chance of the page being placed near its new
> LRU buddies on the next eviction and helps keeping the amount of stale
> swap cache entries low.
> 
> try_to_free_swap() inherently only succeeds when the last user of the
> swap slot vanishes so it is safe to use from places where that single
> mapping just brought the page back to life.
> 

Seems that this has a risk of worsening swap fragmentation for some
situations.  Or not, I have no way of knowing, really.

> diff --git a/mm/memory.c b/mm/memory.c
> index 8b4e40e..407ebf7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2671,8 +2671,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	mem_cgroup_commit_charge_swapin(page, ptr);
>  
>  	swap_free(entry);
> -	if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> -		try_to_free_swap(page);
> +	try_to_free_swap(page);
>  	unlock_page(page);
>  
>  	if (write_access) {
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 621708f..2f0549d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -788,7 +788,7 @@ cull_mlocked:
>  
>  activate_locked:
>  		/* Not a candidate for swapping, so reclaim swap space. */
> -		if (PageSwapCache(page) && vm_swap_full())
> +		if (PageSwapCache(page))
>  			try_to_free_swap(page);
>  		VM_BUG_ON(PageActive(page));
>  		SetPageActive(page);

How are we to know that this is a desirable patch for Linux??