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

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Date:   Wed, 27 Jun 2018 13:05:27 +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,
        Huaisheng Ye <yehs2007@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm: drop VM_BUG_ON from __get_free_pages

On Wed 27-06-18 12:47:39, Vlastimil Babka wrote:
> On 06/27/2018 09:54 AM, Michal Hocko wrote:
> > 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?
> 
> I guess gfp_zone() would have to be checked, dunno about the rewrite of
> GFP_ZONE_TABLE (CCing people).
> In general checks like "if (flags & __GFP_HIGHMEM)" would become false,
> which probably should not be a problem, unless something expect the flag
> to be there and errors out if it isn't.

Well, __GFP_HIGHMEM should be basically GFP_KERNEL for !highmem systems.
But most checks I have seen try to mask it off. Having it 0 would help
to reduce at least some code.

-- 
Michal Hocko
SUSE Labs