linux-kernel - Re: [PATCH] x86/mm/init: respect memblock reserved regions when destroying mappings

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Message-ID: <4D4CA568.70907@goop.org>
Date:	Fri, 04 Feb 2011 17:18:32 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH] x86/mm/init: respect memblock reserved regions when destroying
 mappings

On 02/04/2011 03:35 AM, Stefano Stabellini wrote:
> On Thu, 3 Feb 2011, H. Peter Anvin wrote:
>> On 02/03/2011 03:25 AM, Stefano Stabellini wrote:
>>>> How on Earth would you end up with a reserved region *inside the BRK*?
>>> I think in practice you cannot, but you can have reserved regions at
>>> _end, that is the main problem I am trying to solve.
>>> If we have a reserved region at _end and _end is not PMD aligned, then
>>> we have a problem.
>>>
>>> I thought that checking for reserved regions before destroying the
>>> mapping would be a decent solution (because it wouldn't affect the
>>> normal case); so I ended up checking between _brk_end and _end too.
>>>
>>> Other alternative solutions I thought about but that I discarded because
>>> they also affect the normal case are:
>>>
>>> - never destroy mappings that could go over _end;
>>> - always PMD align _end.
>>>
>>> If none of the above are acceptable, I welcome other suggestions :-)
>>>
>> Sounds like the code does the right thing, but the description needs to
>> be improved.
>>
> I tried to improve both the commit message and the comments within the
> code, this is the result:
>
>
> commit d0136be7b48953f27202dbde285a7379d06cfe98
> Author: Stefano Stabellini <stefano.stabellini@...citrix.com>
> Date:   Tue Jan 25 12:05:11 2011 +0000
>
>     x86/mm/init: respect memblock reserved regions when destroying mappings
>     
>     In init_memory_mapping we destroy the mappings between _brk_end and
>     _end, but if _end is not PMD aligned we also destroy mappings for
>     potentially reserved regions between _end and the following PMD.
>     
>     In order to avoid this problem, before clearing any PMDs we check if the
>     corresponding memory area has been reserved and we only destroy the
>     mapping if hasn't.
>     
>     We found this issue because under Xen we have a valid mapping at _end,
>     and if _end is not PMD aligned the current code destroys the initial
>     part of it.

This description makes more sense, even if the code does exactly the
same thing.

>     
>     In practice this fix does not have any impact on native.
>     
>     Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
>
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 947f42a..65c34f4 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -283,6 +283,8 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
>  	if (!after_bootmem && !start) {
>  		pud_t *pud;
>  		pmd_t *pmd;
> +		unsigned long addr;
> +		u64 size, memblock_addr;
>  
>  		mmu_cr4_features = read_cr4();
>  
> @@ -291,11 +293,22 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
>  		 * located on different 2M pages. cleanup_highmap(), however,
>  		 * can only consider _end when it runs, so destroy any
>  		 * mappings beyond _brk_end here.
> +		 *
> +		 * If _end is not PMD aligned, we also destroy the mapping of
> +		 * the memory area between _end the next PMD, so before clearing
> +		 * the PMD we make sure that the corresponding memory region has
> +		 * not been reserved.
>  		 */
>  		pud = pud_offset(pgd_offset_k(_brk_end), _brk_end);
>  		pmd = pmd_offset(pud, _brk_end - 1);
> -		while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1))
> -			pmd_clear(pmd);
> +		addr = (_brk_end + PMD_SIZE - 1) & PMD_MASK;

I guess its OK if this is >_end, because the pmd offset will be greater
than _end's.  But is there an edge condition if the pmd_offset goes off
the end of the pud, and pud page itself happens to be at the end of the
address space and it wraps?

> +		while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1)) {
Could _end be in a different pud from _brk_end?  Could this walk off the
pud page?

Or is it moot, and there's a guarantee that the whole space is mapped
out of the same pud page?  I guess the original code has the same
concern, so this patch leaves the status quo unchanged.

    J


> +			memblock_addr = memblock_x86_find_in_range_size(__pa(addr),
> +					&size, PMD_SIZE);
> +			if (memblock_addr == (u64) __pa(addr) && size >= PMD_SIZE)
> +				pmd_clear(pmd);
> +			addr += PMD_SIZE;
> +		}
>  	}
>  #endif
>  	__flush_tlb_all();

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