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

[Daniel Vacek]

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???

-nX

>> ------------------------
>> include/linux/mmzone.h:
>> pfn_valid(pfn)
>>   valid_section(__nr_to_section(pfn_to_section_nr(pfn)))
>>     return (section && (section->section_mem_map & SECTION_HAS_MEM_MAP))
>>
>> arch/arm64/mm/init.c:
>> #ifdef CONFIG_HAVE_ARCH_PFN_VALID
>> int pfn_valid(unsigned long pfn)
>> {
>>     return memblock_is_map_memory(pfn << PAGE_SHIFT);
>> }
>> EXPORT_SYMBOL(pfn_valid);
>> #endif
>> ------------------------
>>
>> Also I already sent a fix to Andrew yesterday which was reported to
>> fix the loop.
>>
>> Moreover, you also reported this:
>>
>>>    Early memory node ranges
>>>      node   0: [mem 0x0000000080000000-0x00000000febeffff]
>>>      node   0: [mem 0x00000000febf0000-0x00000000fefcffff]
>>>      node   0: [mem 0x00000000fefd0000-0x00000000ff43ffff]
>>>      node   0: [mem 0x00000000ff440000-0x00000000ff7affff]
>>>      node   0: [mem 0x00000000ff7b0000-0x00000000ffffffff]
>>>      node   0: [mem 0x0000000880000000-0x0000000fffffffff]
>>>    Initmem setup node 0 [mem 0x0000000080000000-0x0000000fffffffff]
>>>    pfn:febf0 oldnext:febf0 newnext:fe9ff
>>>    pfn:febf0 oldnext:febf0 newnext:fe9ff
>>>    pfn:febf0 oldnext:febf0 newnext:fe9ff
>>>    etc etc
>>
>> I am wondering how come pfn_valid(0xfebf0) returns false here. Should
>> it be true or do I miss something?
>>
>> --nX