Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

[Daniel Vacek]

On Thu, Mar 15, 2018 at 4:17 PM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 15 March 2018 at 15:12, Daniel Vacek <neelx@xxxxxxxxxx> wrote:
>> On Thu, Mar 15, 2018 at 8:45 AM, Ard Biesheuvel
>> <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> On 15 March 2018 at 07:44, Daniel Vacek <neelx@xxxxxxxxxx> wrote:
>>>> On Thu, Mar 15, 2018 at 8:36 AM, Ard Biesheuvel
>>>> <ard.biesheuvel@xxxxxxxxxx> wrote:
>>>>> On 15 March 2018 at 02:23, Daniel Vacek <neelx@xxxxxxxxxx> wrote:
>>>>>> On Wed, Mar 14, 2018 at 8:29 PM, Ard Biesheuvel
>>>>>> <ard.biesheuvel@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>>>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>>>>>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>>>>>> Cc: Paul Burton <paul.burton@xxxxxxxxxx>
>>>>>>> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>>>>>>> Cc: Vlastimil Babka <vbabka@xxxxxxx>
>>>>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>>>>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>>>>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>>>>>>> ---
>>>>>>>  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*?

abstractions?

> 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.

If you don't like the name I would argue it should be changed to
something like memblock_pfn_of_next_memory(). Still the name is not
next_valid_pfn() but memblock_next... kind of distinguishing something
different.

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

I'm trying to learn. Hence I was asking what is the abstract meaning
of it. As I see two *way_different* implementations so I am not sure
how I should understand that.

>> 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.

Cool. I'll do that next week. Thank you, sir.

--nX