Re: [PATCH] hugetlb: fix uninitialized subpool pointer

[Michal Hocko]

On Tue 23-02-21 13:55:44, Mike Kravetz wrote:
> Gerald Schaefer reported a panic on s390 in hugepage_subpool_put_pages()
> with linux-next 5.12.0-20210222.
> Call trace:
>   hugepage_subpool_put_pages.part.0+0x2c/0x138
>   __free_huge_page+0xce/0x310
>   alloc_pool_huge_page+0x102/0x120
>   set_max_huge_pages+0x13e/0x350
>   hugetlb_sysctl_handler_common+0xd8/0x110
>   hugetlb_sysctl_handler+0x48/0x58
>   proc_sys_call_handler+0x138/0x238
>   new_sync_write+0x10e/0x198
>   vfs_write.part.0+0x12c/0x238
>   ksys_write+0x68/0xf8
>   do_syscall+0x82/0xd0
>   __do_syscall+0xb4/0xc8
>   system_call+0x72/0x98
> 
> This is a result of the change which moved the hugetlb page subpool
> pointer from page->private to page[1]->private.  When new pages are
> allocated from the buddy allocator, the private field of the head
> page will be cleared, but the private field of subpages is not modified.
> Therefore, old values may remain.

Very interesting. I have expected that the page->private would be in a
reasonable state when allocated. On the other hand hugetlb doesn't do
__GFP_COMP so tail pages are not initialized by the allocator.
 
> Fix by initializing hugetlb page subpool pointer in prep_new_huge_page().
> 
> Fixes: f1280272ae4d ("hugetlb: use page.private for hugetlb specific page flags")

This is not a stable sha to refer to as it comes from linux next.

> Reported-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

I think this would be worth a separate patch rather than having it
folded into the original patch. Thi is really subtle.

> ---
>  mm/hugetlb.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c232cb67dda2..7ae5c18c98a7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1465,6 +1465,7 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>  {
>  	INIT_LIST_HEAD(&page->lru);
>  	set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> +	hugetlb_set_page_subpool(page, NULL);
>  	set_hugetlb_cgroup(page, NULL);
>  	set_hugetlb_cgroup_rsvd(page, NULL);
>  	spin_lock(&hugetlb_lock);
> -- 
> 2.29.2
> 

-- 
Michal Hocko
SUSE Labs