Question on follow_page_mask

[Kirill A. Shutemov]

On Tue, Feb 23, 2016 at 06:45:05PM +0530, Anshuman Khandual wrote:
> Not able to understand the first code block of follow_page_mask
> function. follow_huge_addr function is expected to find the page
> struct for the given address if it turns out to be a HugeTLB page
> but then when it finds the page we bug on if it had been called
> with FOLL_GET flag.
> 
> 	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
> 	if (!IS_ERR(page)) {
> 		BUG_ON(flags & FOLL_GET);
> 		return page;
> 	}
> 
> do_move_page_to_node_array calls follow_page with FOLL_GET which
> in turn calls follow_page_mask with FOLL_GET. On POWER, the
> function follow_huge_addr is defined and does not return -EINVAL
> like the generic one. It returns the page struct if its a HugeTLB
> page. Just curious to know what is the purpose behind the BUG_ON.

I would guess requesting pin on non-reclaimable page is considered
useless, meaning suspicius behavior. BUG_ON() is overkill, I think.
WARN_ON_ONCE() would make it.

Not that this follow_huge_addr() on Power is not reachable via
do_move_page_to_node_array(), because the vma is !vma_is_migratable().

-- 
 Kirill A. Shutemov