Re: mm: how to check for kernel pages

[Michal Hocko]

On Sun, Feb 04, 2007 at 02:57:30PM +0100, Arjan van de Ven wrote:
> On Sun, 2007-02-04 at 12:02 +0100, Michal Hocko wrote:
> > Hi,
> > is there any effective and fast way how to find out whether page
> > given by its page frame number is currenly used by (mapped by) kernel?
> 
> what do you want to use this for? The answer to your question greatly
> depends on that...

Sorry for not being more precise. As a part of my thesis work I need to
migrate pages. I greatly use mm/migrate.c code.  
I assume that not all pages can be migrated - especially those used by
kernel (where direct virtual to physical mapping is used).
So I intended something like following code:

/* Returns 0 if page is suitable to be migration source */
int check_migrate_page(struct page * page)
{
	return PageBuddy(page) || PageCompound(page) || ... 
		kernel_page(page);
}

/* Steals pfn page from its current user(s) and replace it by newly
 * allocated page. pfn page structure is returned if page was migrated
 * to new location (can be reused by caller) or NULL otherwise.
 */
struct page * steal_page(unsigned long pfn)
{
	int ret;
	struct page *page, *target_page;

	page = pfn_to_page(pfn);
	if(!page)
		goto bad_page;

	ret = check_migrate_page(page);
	if(ret)
		goto bad_page;

	target_page = alloc_pages(flags, 0);
	/* uses migrate_pages function internally */
	ret = do_migration(page, target_page);
	if(ret)
		goto bad_page;

	/* TODO clear page frame data. */
	return page;
bad_page:
	return NULL;
}

Please add me to Cc, because I am not the list member.
Best regards
-- 
Michal Hocko

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