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

[Ard Biesheuvel]

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.

> ------------------------
> 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