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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Thu, 15 Mar 2018 15:17:19 +0000
Message-ID: <CAKv+Gu8dvCRUnS5KxDWa4GP2vMPo9cUWVNybx+tGY_Y8T67Uvw@mail.gmail.com>
Subject: Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"
To:     Daniel Vacek <neelx@redhat.com>
Cc:     linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>,
        Paul Burton <paul.burton@imgtec.com>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On 15 March 2018 at 15:12, Daniel Vacek <neelx@redhat.com> wrote:
> On Thu, Mar 15, 2018 at 8:45 AM, Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
>> On 15 March 2018 at 07:44, Daniel Vacek <neelx@redhat.com> wrote:
>>> On Thu, Mar 15, 2018 at 8:36 AM, Ard Biesheuvel
>>> <ard.biesheuvel@linaro.org> wrote:
>>>> On 15 March 2018 at 02:23, Daniel Vacek <neelx@redhat.com> wrote:
>>>>> On Wed, Mar 14, 2018 at 8:29 PM, Ard Biesheuvel
>>>>> <ard.biesheuvel@linaro.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@redhat.com>
>>>>>> Cc: Mel Gorman <mgorman@techsingularity.net>
>>>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>>>> Cc: Paul Burton <paul.burton@imgtec.com>
>>>>>> Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
>>>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>>>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>>>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.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?
>

Who cares what it *means*?

memblock_next_valid_pfn() has 'valid_pfn' in its name, so if passing
pfn A returns B, and there exists a C such that A < C < B and
pfn_valid(C) returns true, memblock_next_valid_pfn doesn't do what it
says on the tin and should be fixed.

You keep going on about how pfn_valid() does or does not do what you
think, but that is really irrelevant.

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

OK. You can add my ack to a patch that reverts it, and we can revisit
it for the next cycle.