Re: [rfc 1/3] mm: vmscan: never swap under low memory pressure

[KAMEZAWA Hiroyuki]

On Wed, 2 Nov 2011 17:31:41 +0100
Johannes Weiner <jweiner@xxxxxxxxxx> wrote:

> We want to prevent floods of used-once file cache pushing us to swap
> out anonymous pages.  Never swap under a certain priority level.  The
> availability of used-once cache pages should prevent us from reaching
> that threshold.
> 
> This is needed because subsequent patches will revert some of the
> mechanisms that tried to prefer file over anon, and this should not
> result in more eager swapping again.
> 
> It might also be better to keep the aging machinery going and just not
> swap, rather than staying away from anonymous pages in the first place
> and having less useful age information at the time of swapout.
> 
> Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> ---
>  mm/vmscan.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a90c603..39d3da3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -831,6 +831,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		 * Try to allocate it some swap space here.
>  		 */
>  		if (PageAnon(page) && !PageSwapCache(page)) {
> +			if (priority >= DEF_PRIORITY - 2)
> +				goto keep_locked;
>  			if (!(sc->gfp_mask & __GFP_IO))
>  				goto keep_locked;
>  			if (!add_to_swap(page))

Hm, how about not scanning LRU_ANON rather than checking here ?
Add some bias to get_scan_count() or some..
If you think to need rotation of LRU, only kswapd should do that..

Thanks,
-Kame

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