Re: [patch]vmscan: protect exectuable page from inactive list scan

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Sep 29, 2010 at 10:57:40AM +0800, Shaohua Li wrote:
> With commit 645747462435, pte referenced file page isn't activated in inactive
> list scan. For VM_EXEC page, if it can't get a chance to active list, the
> executable page protect loses its effect. We protect such page in inactive scan
> here, now such page will be guaranteed cached in a full scan of active and
> inactive list, which restores previous behavior.

This change was in the back of my head since the used-once detection
was merged but there were never any regressions reported that would
indicate a requirement for it.

Does this patch fix a problem you observed?

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -608,8 +608,15 @@ static enum page_references page_check_references(struct page *page,
>  		 * quickly recovered.
>  		 */
>  		SetPageReferenced(page);
> -
> -		if (referenced_page)
> +		/*
> +		 * Identify pte referenced and file-backed pages and give them
> +		 * one trip around the active list. So that executable code get
> +		 * better chances to stay in memory under moderate memory
> +		 * pressure. JVM can create lots of anon VM_EXEC pages, so we
> +		 * ignore them here.

PTE-referenced PageAnon() pages are activated unconditionally a few
lines further up, so the page_is_file_cache() check filters only shmem
pages.  I doubt this was your intention...?

> +		 */
> +		if (referenced_page || ((vm_flags & VM_EXEC) &&
> +		    page_is_file_cache(page)))
>  			return PAGEREF_ACTIVATE;
>  
>  		return PAGEREF_KEEP;

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