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

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Message-ID: <20180626144456.GZ28965@dhcp22.suse.cz>
Date:   Tue, 26 Jun 2018 16:44:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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
Subject: Re: [PATCH] mm: drop VM_BUG_ON from __get_free_pages

On Tue 26-06-18 15:57:39, Vlastimil Babka wrote:
> 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?

got lost somewhere on the way during the discussion. Thanks for spotting
that.

Andrew, could you add gfp_mask & ~__GFP_HIGHMEM please? Or should I
resubmit?

-- 
Michal Hocko
SUSE Labs