Re: [BUG] __GFP_THISNODE is not always honored

Mel Gorman
2008-08-18T10:59:18Z

On (15/08/08 17:01), Adam Litke didst pronounce:
> While running the libhugetlbfs test suite on a NUMA machine with 2.6.27-rc3, I
> discovered some strange behavior with __GFP_THISNODE. The hugetlb function
> alloc_fresh_huge_page_node() calls alloc_pages_node() with __GFP_THISNODE but
> occasionally a page that is not on the requested node is returned.

That's bad in itself and has wider reaching consequences than hugetlb
getting its counters wrong. I believe SLUB depends on __GFP_THISNODE
being obeyed for example. Can you boot the machine in question with
mminit_loglevel=4 and loglevel=8 set on the command line and send me the
dmesg please? It should output the zonelists and I might be able to
figure out what's going wrong. Thanks

> Since the
> hugetlb code assumes that the page will be on the requested node, badness follows
> when the page is added to the wrong node's free_list.
>
> There is clearly something wrong with the buddy allocator since __GFP_THISNODE
> cannot be trusted. Until that is fixed, the hugetlb code should not assume
> that the newly allocated page is on the node asked for. This patch prevents
> the hugetlb pool counters from being corrupted and allows the code to cope with
> unbalanced numa allocations.
>
> So far my debugging has led me to get_page_from_freelist() inside the
> for_each_zone_zonelist() loop. When buffered_rmqueue() returns a page I
> compare the value of page_to_nid(page), zone->node and the node that the
> hugetlb code requested with __GFP_THISNODE. These all match -- except when the
> problem triggers. In that case, zone->node matches the node we asked for but
> page_to_nid() does not.
>

Feels like the wrong zonelist is being used. The dmesg with
mminit_loglevel may tell.

> Workaround patch:
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 67a7119..7a30a61 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -568,7 +568,7 @@ static struct page *alloc_fresh_huge_page_node(struct hstate *h, int nid)
> __free_pages(page, huge_page_order(h));
> return NULL;
> }
> - prep_new_huge_page(h, page, nid);
> + prep_new_huge_page(h, page, page_to_nid(page));
> }

This will mask the bug for hugetlb but I wonder if this should be a
VM_BUG_ON(page_to_nid(page) != nid) ?

>
> return page;
>
> --
> Adam Litke - (agl at us.ibm.com)
> IBM Linux Technology Center
>

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab