linux-kernel - Re: [PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

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Message-ID: <CAL01qpvbDRnE0mHBttomcqYtT6i9OaG_kvnj6BMXtqYn4cP1FQ@mail.gmail.com>
Date:	Tue, 3 Jun 2014 16:17:06 +0100
From:	"Fleming, Matt" <matt.fleming@...el.com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

On 27 May 2014 19:45, Dave Hansen <dave.hansen@...el.com> wrote:
>
> I don't think this is quite right.  pfn_valid() tells us whether we have
> a 'struct page' there or not.  *BUT*, it does not tell us whether it is
> RAM that we can actually address and than can be freed in to the buddy
> allocator.
>
> I think sparsemem is where this matters.  Let's say mem= caused lowmem
> to end in the middle of a section (or that 896MB wasn't
> section-aligned).  Then someone calls free_bootmem_late() on an area
> that is in the last section, but _above_ max_mapnr.  It'll be
> pfn_valid(), we'll free it in to the buddy allocator, and we'll blam the
> first time we try to write to a bogus vaddr after a phys_to_virt().

Ah, the sparsemem case wasn't something I'd considered. Thanks Dave.

> At a higher level, I don't like the idea of the bootmem code papering
> over bugs when somebody calls in to it trying to _free_ stuff that's not
> memory (as far as the kernel is concerned).
>
> I think the right thing to do is to call in to the e820 code and see if
> the range is E820_RAM before trying to bootmem-free it.

OK, this makes sense. I'll try that approach and see if it also fixes
Alan's problem.
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