linux-kernel - Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init

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Date:   Thu, 15 Mar 2018 16:12:20 +0100
From:   Daniel Vacek <neelx@...hat.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Paul Burton <paul.burton@...tec.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

On Thu, Mar 15, 2018 at 8:45 AM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 15 March 2018 at 07:44, Daniel Vacek <neelx@...hat.com> wrote:
>> On Thu, Mar 15, 2018 at 8:36 AM, Ard Biesheuvel
>> <ard.biesheuvel@...aro.org> wrote:
>>> On 15 March 2018 at 02:23, Daniel Vacek <neelx@...hat.com> wrote:
>>>> On Wed, Mar 14, 2018 at 8:29 PM, Ard Biesheuvel
>>>> <ard.biesheuvel@...aro.org> wrote:
>>>>> This reverts commit 864b75f9d6b0100bb24fdd9a20d156e7cda9b5ae.
>>>>>
>>>>> Commit 864b75f9d6b0 ("mm/page_alloc: fix memmap_init_zone pageblock
>>>>> alignment") modified the logic in memmap_init_zone() to initialize
>>>>> struct pages associated with invalid PFNs, to appease a VM_BUG_ON()
>>>>> in move_freepages(), which is redundant by its own admission, and
>>>>> dereferences struct page fields to obtain the zone without checking
>>>>> whether the struct pages in question are valid to begin with.
>>>>>
>>>>> Commit 864b75f9d6b0 only makes it worse, since the rounding it does
>>>>> may cause pfn assume the same value it had in a prior iteration of
>>>>> the loop, resulting in an infinite loop and a hang very early in the
>>>>> boot. Also, since it doesn't perform the same rounding on start_pfn
>>>>> itself but only on intermediate values following an invalid PFN, we
>>>>> may still hit the same VM_BUG_ON() as before.
>>>>>
>>>>> So instead, let's fix this at the core, and ensure that the BUG
>>>>> check doesn't dereference struct page fields of invalid pages.
>>>>>
>>>>> Fixes: 864b75f9d6b0 ("mm/page_alloc: fix memmap_init_zone pageblock alignment")
>>>>> Cc: Daniel Vacek <neelx@...hat.com>
>>>>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>>>>> Cc: Michal Hocko <mhocko@...e.com>
>>>>> Cc: Paul Burton <paul.burton@...tec.com>
>>>>> Cc: Pavel Tatashin <pasha.tatashin@...cle.com>
>>>>> Cc: Vlastimil Babka <vbabka@...e.cz>
>>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>>>>> ---
>>>>>  mm/page_alloc.c | 13 +++++--------
>>>>>  1 file changed, 5 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>> index 3d974cb2a1a1..635d7dd29d7f 100644
>>>>> --- a/mm/page_alloc.c
>>>>> +++ b/mm/page_alloc.c
>>>>> @@ -1910,7 +1910,9 @@ static int move_freepages(struct zone *zone,
>>>>>          * Remove at a later date when no bug reports exist related to
>>>>>          * grouping pages by mobility
>>>>>          */
>>>>> -       VM_BUG_ON(page_zone(start_page) != page_zone(end_page));
>>>>> +       VM_BUG_ON(pfn_valid(page_to_pfn(start_page)) &&
>>>>> +                 pfn_valid(page_to_pfn(end_page)) &&
>>>>> +                 page_zone(start_page) != page_zone(end_page));
>>>>
>>>> Hi, I am on vacation this week and I didn't have a chance to test this
>>>> yet but I am not sure this is correct. Generic pfn_valid() unlike the
>>>> arm{,64} arch specific versions returns true for all pfns in a section
>>>> if there is at least some memory mapped in that section. So I doubt
>>>> this prevents the crash I was targeting. I believe pfn_valid() does
>>>> not change a thing here :(
>>>>
>>>
>>> If this is the case, memblock_next_valid_pfn() is broken since it
>>> skips valid PFNs, and we should be fixing that instead.
>>
>> How do you define valid pfn? Maybe the generic version of pfn_valid()
>> should be fixed???
>>
>
> memblock_next_valid_pfn() skips PFNs for which pfn_valid() returns
> true. That is clearly a bug.

So pfn_valid() does not mean this frame is usable memory?

OK, in that case we need to fix or revert memblock_next_valid_pfn().
That works for me.

--nX