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

[Daniel Vacek]

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 :(

------------------------
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