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

From: "Kirill A. Shutemov" <[email-masked]>
Date: Thu, 2 Jan 2014 04:19:51 +0200

On Wed, Jan 01, 2014 at 09:29:05PM +1100, Benjamin Herrenschmidt wrote:
> 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 ?

Looks sane to me. Or something with IS_ENABLED(), if needed.

> 
> 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.
> > -			 */

Please don't lose the comment.

> > -			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:
> 
> 
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-- 
 Kirill A. Shutemov