On Wed, Mar 14, 2018 at 07:29:37PM +0000, Ard Biesheuvel wrote:
> This reverts commit 864b75f9d6b0100bb24fdd9a20d156e7cda9b5ae.
FWIW, the revert fixes the boot hang I'm seeing on ThunderX1.
--Jan
> 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 <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Paul Burton <[email protected]>
> Cc: Pavel Tatashin <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> Signed-off-by: Ard Biesheuvel <[email protected]>
> ---
> 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));
> #endif
>
> if (num_movable)
> @@ -5359,14 +5361,9 @@ void __meminit memmap_init_zone(unsigned long size,
> int nid, unsigned long zone,
> /*
> * Skip to the pfn preceding the next valid one (or
> * end_pfn), such that we hit a valid pfn (or end_pfn)
> - * on our next iteration of the loop. Note that it needs
> - * to be pageblock aligned even when the region itself
> - * is not. move_freepages_block() can shift ahead of
> - * the valid region but still depends on correct page
> - * metadata.
> + * on our next iteration of the loop.
> */
> - pfn = (memblock_next_valid_pfn(pfn, end_pfn) &
> - ~(pageblock_nr_pages-1)) - 1;
> + pfn = memblock_next_valid_pfn(pfn, end_pfn) - 1;
> #endif
> continue;
> }
> --
> 2.15.1
>
>
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