Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM)

From: Martin Schwidefsky <[email-masked]>
Date: Wed, 24 Feb 2016 09:39:15 +0100

On Tue, 23 Feb 2016 22:33:45 +0300
"Kirill A. Shutemov" <[email-masked]> wrote:

> On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote:
> > I'll check with Martin, maybe it is actually trivial, then we can
> > do a quick test it to rule that one out.
> 
> Oh. I found a bug in __split_huge_pmd_locked(). Although, not sure if it's
> _the_ bug.
> 
> pmdp_invalidate() is called for the wrong address :-/
> I guess that can be destructive on the architecture, right?
> 
> Could you check this?
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1c317b85ea7d..4246bc70e55a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2865,7 +2865,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
>  	pmd_populate(mm, &_pmd, pgtable);
> 
> -	for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> +	for (i = 0; i < HPAGE_PMD_NR; i++) {
>  		pte_t entry, *pte;
>  		/*
>  		 * Note that NUMA hinting access restrictions are not
> @@ -2886,9 +2886,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		}
>  		if (dirty)
>  			SetPageDirty(page + i);
> -		pte = pte_offset_map(&_pmd, haddr);
> +		pte = pte_offset_map(&_pmd, haddr + i * PAGE_SIZE);
>  		BUG_ON(!pte_none(*pte));
> -		set_pte_at(mm, haddr, pte, entry);
> +		set_pte_at(mm, haddr + i * PAGE_SIZE, pte, entry);
>  		atomic_inc(&page[i]._mapcount);
>  		pte_unmap(pte);
>  	}
> @@ -2938,7 +2938,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	pmd_populate(mm, pmd, pgtable);
> 
>  	if (freeze) {
> -		for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> +		for (i = 0; i < HPAGE_PMD_NR; i++) {
>  			page_remove_rmap(page + i, false);
>  			put_page(page + i);
>  		}

Test is running and it looks good so far. For the final assessment I defer
to Gerald and Sebastian.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.