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

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Message-ID: <20180627075403.GG32348@dhcp22.suse.cz>
Date:   Wed, 27 Jun 2018 09:54:03 +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 Wed 27-06-18 09:50:01, Vlastimil Babka wrote:
> On 06/27/2018 09:34 AM, Michal Hocko wrote:
> > On Tue 26-06-18 10:04:16, Andrew Morton wrote:
> > 
> > And as I've argued before the code would be wrong regardless. We would
> > leak the memory or worse touch somebody's else kmap without knowing
> > that.  So we have a choice between a mem leak, data corruption k or a
> > silent fixup. I would prefer the last option. And blowing up on a BUG
> > is not much better on something that is easily fixable. I am not really
> > convinced that & ~__GFP_HIGHMEM is something to lose sleep over.
> 
> Maybe put the fixup into a "#ifdef CONFIG_HIGHMEM" block and then modern
> systems won't care? In that case it could even be if (WARN_ON_ONCE(...))
> so future cases with wrong expectations would become known.

Yes that could be done as well. Or maybe we can make __GFP_HIGHMEM 0 for
!HIGHMEM systems. Does something really rely on it being non-zero?

-- 
Michal Hocko
SUSE Labs