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

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Message-ID: <AANLkTi=nEQka_19+nKQC-yJn2Czn9GCqsUWN6Byy7rCF@mail.gmail.com>
Date:	Sat, 5 Feb 2011 23:02:43 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"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 Fri, Feb 4, 2011 at 5:18 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 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();

why not just move calling cleanup_highmap down?

something like attached patch.

View attachment "fix_cleanup_highmap.patch" of type "text/x-patch" (3660 bytes)