Re: [PATCH v2 1/3] mm/shuffle: don't move pages between zones and don't read garbage memmaps

Wei Yang
2020-06-22T08:26:45Z

On Fri, Jun 19, 2020 at 02:59:20PM +0200, David Hildenbrand wrote:
>Especially with memory hotplug, we can have offline sections (with a
>garbage memmap) and overlapping zones. We have to make sure to only
>touch initialized memmaps (online sections managed by the buddy) and that
>the zone matches, to not move pages between zones.
>
>To test if this can actually happen, I added a simple
> BUG_ON(page_zone(page_i) != page_zone(page_j));
>right before the swap. When hotplugging a 256M DIMM to a 4G x86-64 VM and
>onlining the first memory block "online_movable" and the second memory
>block "online_kernel", it will trigger the BUG, as both zones (NORMAL
>and MOVABLE) overlap.
>
>This might result in all kinds of weird situations (e.g., double
>allocations, list corruptions, unmovable allocations ending up in the
>movable zone).
>
>Fixes: e900a918b098 ("mm: shuffle initial free memory to improve memory-side-cache utilization")
>Acked-by: Michal Hocko <mhocko@suse.com>
>Cc: stable@vger.kernel.org # v5.2+
>Cc: Andrew Morton <akpm@linux-foundation.org>
>Cc: Johannes Weiner <hannes@cmpxchg.org>
>Cc: Michal Hocko <mhocko@suse.com>
>Cc: Minchan Kim <minchan@kernel.org>
>Cc: Huang Ying <ying.huang@intel.com>
>Cc: Wei Yang <richard.weiyang@gmail.com>
>Cc: Mel Gorman <mgorman@techsingularity.net>
>Signed-off-by: David Hildenbrand <david@redhat.com>
>---
> mm/shuffle.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
>diff --git a/mm/shuffle.c b/mm/shuffle.c
>index 44406d9977c77..dd13ab851b3ee 100644
>--- a/mm/shuffle.c
>+++ b/mm/shuffle.c
>@@ -58,25 +58,25 @@ module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);
> * For two pages to be swapped in the shuffle, they must be free (on a
> * 'free_area' lru), have the same order, and have the same migratetype.
> */
>-static struct page * __meminit shuffle_valid_page(unsigned long pfn, int order)
>+static struct page * __meminit shuffle_valid_page(struct zone *zone,
>+ unsigned long pfn, int order)
> {
>- struct page *page;
>+ struct page *page = pfn_to_online_page(pfn);

Hi, David and Dan,

One thing I want to confirm here is we won't have partially online section,
right? We can add a sub-section to system, but we won't manage it by buddy.

With this confirmed:

Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>

>
> /*
> * Given we're dealing with randomly selected pfns in a zone we
> * need to ask questions like...
> */
>
>- /* ...is the pfn even in the memmap? */
>- if (!pfn_valid_within(pfn))
>+ /* ... is the page managed by the buddy? */
>+ if (!page)
> return NULL;
>
>- /* ...is the pfn in a present section or a hole? */
>- if (!pfn_in_present_section(pfn))
>+ /* ... is the page assigned to the same zone? */
>+ if (page_zone(page) != zone)
> return NULL;
>
> /* ...is the page free and currently on a free_area list? */
>- page = pfn_to_page(pfn);
> if (!PageBuddy(page))
> return NULL;
>
>@@ -123,7 +123,7 @@ void __meminit __shuffle_zone(struct zone *z)
> * page_j randomly selected in the span @zone_start_pfn to
> * @spanned_pages.
> */
>- page_i = shuffle_valid_page(i, order);
>+ page_i = shuffle_valid_page(z, i, order);
> if (!page_i)
> continue;
>
>@@ -137,7 +137,7 @@ void __meminit __shuffle_zone(struct zone *z)
> j = z->zone_start_pfn +
> ALIGN_DOWN(get_random_long() % z->spanned_pages,
> order_pages);
>- page_j = shuffle_valid_page(j, order);
>+ page_j = shuffle_valid_page(z, j, order);
> if (page_j && page_j != page_i)
> break;
> }
>--
>2.26.2

--
Wei Yang
Help you, Help me