Re: [PATCH] mm: drop VM_BUG_ON from __get_free_pages

From: Vlastimil Babka
Date: Tue Jun 26 2018 - 09:57:49 EST


On 06/22/2018 06:28 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> There is no real reason to blow up just because the caller doesn't know
> that __get_free_pages cannot return highmem pages. Simply fix that up
> silently. Even if we have some confused users such a fixup will not be
> harmful.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> Hi Andrew,
> previously posted [1] but it fell through cracks. Can we merge it now?
>
> [1] http://lkml.kernel.org/r/20171129160446.jluzpv3n6mjc3fwv@xxxxxxxxxxxxxx
>
> mm/page_alloc.c | 10 +++-------
> 1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1521100f1e63..5f56f662a52d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4402,18 +4402,14 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> EXPORT_SYMBOL(__alloc_pages_nodemask);
>
> /*
> - * Common helper functions.
> + * Common helper functions. Never use with __GFP_HIGHMEM because the returned
> + * address cannot represent highmem pages. Use alloc_pages and then kmap if
> + * you need to access high mem.
> */
> unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
> {
> struct page *page;
>
> - /*
> - * __get_free_pages() returns a virtual address, which cannot represent
> - * a highmem page
> - */
> - VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
> -
> page = alloc_pages(gfp_mask, order);

The previous version had also replaced the line above with:

+ page = alloc_pages(gfp_mask & ~__GFP_HIGHMEM, order);

This one doesn't, yet you say "fix that up silently". Bug?

> if (!page)
> return 0;
>