linux-kernel - Re: [PATCH] mm: drop VM_BUG_ON from __get_free

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Date:   Tue, 26 Jun 2018 15:57:39 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     JianKang Chen <chenjiankang1@...wei.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, xieyisheng1@...wei.com,
        guohanjun@...wei.com, wangkefeng.wang@...wei.com,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: drop VM_BUG_ON from __get_free_pages

On 06/22/2018 06:28 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> 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@...e.com>
> ---
> Hi Andrew,
> previously posted [1] but it fell through cracks. Can we merge it now?
> 
> [1] http://lkml.kernel.org/r/20171129160446.jluzpv3n6mjc3fwv@dhcp22.suse.cz
> 
>  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;
>