Subject: Re: [PATCH] powerpc: thp: Fix crash on mremap

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 01 Jan 2014 21:29:05 +1100

On Wed, 2014-01-01 at 15:23 +0530, Aneesh Kumar K.V wrote:
> From: "Aneesh Kumar K.V" <[email-masked]>
> 
> This patch fix the below crash
> 
> NIP [c00000000004cee4] .__hash_page_thp+0x2a4/0x440
> LR [c0000000000439ac] .hash_page+0x18c/0x5e0
> ...
> Call Trace:
> [c000000736103c40] [00001ffffb000000] 0x1ffffb000000(unreliable)
> [437908.479693] [c000000736103d50] [c0000000000439ac] .hash_page+0x18c/0x5e0
> [437908.479699] [c000000736103e30] [c00000000000924c] .do_hash_page+0x4c/0x58
> 
> On ppc64 we use the pgtable for storing the hpte slot information and
> store address to the pgtable at a constant offset (PTRS_PER_PMD) from
> pmd. On mremap, when we switch the pmd, we need to withdraw and deposit
> the pgtable again, so that we find the pgtable at PTRS_PER_PMD offset
> from new pmd.
> 
> We also want to move the withdraw and deposit before the set_pmd so
> that, when page fault find the pmd as trans huge we can be sure that
> pgtable can be located at the offset.
> 
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
> NOTE:
> For other archs we would just be removing the pgtable from the list and adding it back.
> I didn't find an easy way to make it not do that without lots of #ifdef around. Any
> suggestion around that is welcome.

What about

-		if (new_ptl != old_ptl) {
+               if (new_ptl != old_ptl || ARCH_THP_MOVE_PMD_ALWAYS_WITHDRAW) {

Or something similar ?

Cheers,
Ben.

>  mm/huge_memory.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 7de1bf85f683..eb2e60d9ba45 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1500,24 +1500,23 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
>  	 */
>  	ret = __pmd_trans_huge_lock(old_pmd, vma, &old_ptl);
>  	if (ret == 1) {
> +		pgtable_t pgtable;
> +
>  		new_ptl = pmd_lockptr(mm, new_pmd);
>  		if (new_ptl != old_ptl)
>  			spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
>  		pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
>  		VM_BUG_ON(!pmd_none(*new_pmd));
> +		/*
> +		 * Archs like ppc64 use pgtable to store per pmd
> +		 * specific information. So when we switch the pmd,
> +		 * we should also withdraw and deposit the pgtable
> +		 */
> +		pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
> +		pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
>  		set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
> -		if (new_ptl != old_ptl) {
> -			pgtable_t pgtable;
> -
> -			/*
> -			 * Move preallocated PTE page table if new_pmd is on
> -			 * different PMD page table.
> -			 */
> -			pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
> -			pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
> -
> +		if (new_ptl != old_ptl)
>  			spin_unlock(new_ptl);
> -		}
>  		spin_unlock(old_ptl);
>  	}
>  out: