Re: [PATCH 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare

[Mike Kravetz]

On 12/06/22 11:45, Peter Xu wrote:
> On Mon, Dec 05, 2022 at 03:52:51PM -0800, John Hubbard wrote:
> > On 12/5/22 15:33, Mike Kravetz wrote:
> > > On 11/29/22 14:35, Peter Xu wrote:
> > > > Since walk_hugetlb_range() walks the pgtable, it needs the vma lock
> > > > to make sure the pgtable page will not be freed concurrently.
> > > > 
> > > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > > ---
> > > >   mm/pagewalk.c | 2 ++
> > > >   1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> > > > index 7f1c9b274906..d98564a7be57 100644
> > > > --- a/mm/pagewalk.c
> > > > +++ b/mm/pagewalk.c
> > > > @@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
> > > >   	const struct mm_walk_ops *ops = walk->ops;
> > > >   	int err = 0;
> > > > +	hugetlb_vma_lock_read(vma);
> > > >   	do {
> > > >   		next = hugetlb_entry_end(h, addr, end);
> > > >   		pte = huge_pte_offset(walk->mm, addr & hmask, sz);
> > > 
> > > For each found pte, we will be calling mm_walk_ops->hugetlb_entry() with
> > > the vma_lock held.  I looked into the various hugetlb_entry routines, and
> > > I am not sure about hmm_vma_walk_hugetlb_entry.  It seems like it could
> > > possibly call hmm_vma_fault -> handle_mm_fault -> hugetlb_fault.  If this
> > > can happen, then we may have an issue as hugetlb_fault will also need to
> > > acquire the vma_lock in read mode.
> 
> Thanks for spotting that, Mike.
> 
> I used to notice that path special but that's when I was still using RCU
> locks who doesn't have the issue.  Then I overlooked this one when
> switchover.
> 
> > > 
> > > I do not know the hmm code well enough to know if this may be an actual
> > > issue?
> > 
> > Oh, this sounds like a serious concern. If we add a new lock, and hold it
> > during callbacks that also need to take it, that's not going to work out,
> > right?
> > 
> > And yes, hmm_range_fault() and related things do a good job of revealing
> > this kind of deadlock. :)
> 
> I've got a fixup attached.  John, since this got your attention please also
> have a look too in case there's further issues.
> 
> Thanks,
> 
> -- 
> Peter Xu

Thanks Peter.  I am good with the fixup.  When combined with original,

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz


> From 9ad1e65a31f51a0dc687cd9d6083b9e920d2da61 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@xxxxxxxxxx>
> Date: Tue, 6 Dec 2022 11:38:47 -0500
> Subject: [PATCH] fixup! mm/hugetlb: Make walk_hugetlb_range() safe to pmd
>  unshare
> Content-type: text/plain
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  arch/s390/mm/gmap.c      | 2 ++
>  fs/proc/task_mmu.c       | 2 ++
>  include/linux/pagewalk.h | 8 +++++++-
>  mm/hmm.c                 | 8 +++++++-
>  4 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 8947451ae021..292a54c490d4 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2643,7 +2643,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
>  	end = start + HPAGE_SIZE - 1;
>  	__storage_key_init_range(start, end);
>  	set_bit(PG_arch_1, &page->flags);
> +	hugetlb_vma_unlock_read(walk->vma);
>  	cond_resched();
> +	hugetlb_vma_lock_read(walk->vma);
>  	return 0;
>  }
>  
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 89338950afd3..d7155f3bb678 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1612,7 +1612,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
>  			frame++;
>  	}
>  
> +	hugetlb_vma_unlock_read(walk->vma);
>  	cond_resched();
> +	hugetlb_vma_lock_read(walk->vma);
>  
>  	return err;
>  }
> diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> index 959f52e5867d..1f7c2011f6cb 100644
> --- a/include/linux/pagewalk.h
> +++ b/include/linux/pagewalk.h
> @@ -21,7 +21,13 @@ struct mm_walk;
>   *			depth is -1 if not known, 0:PGD, 1:P4D, 2:PUD, 3:PMD.
>   *			Any folded depths (where PTRS_PER_P?D is equal to 1)
>   *			are skipped.
> - * @hugetlb_entry:	if set, called for each hugetlb entry
> + * @hugetlb_entry:	if set, called for each hugetlb entry.	Note that
> + *			currently the hook function is protected by hugetlb
> + *			vma lock to make sure pte_t* and the spinlock is valid
> + *			to access.  If the hook function needs to yield the
> + *			thread or retake the vma lock for some reason, it
> + *			needs to properly release the vma lock manually,
> + *			and retake it before the function returns.
>   * @test_walk:		caller specific callback function to determine whether
>   *			we walk over the current vma or not. Returning 0 means
>   *			"do page table walk over the current vma", returning
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 3850fb625dda..dcd624f28bcf 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -493,8 +493,14 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
>  	required_fault =
>  		hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
>  	if (required_fault) {
> +		int ret;
> +
>  		spin_unlock(ptl);
> -		return hmm_vma_fault(addr, end, required_fault, walk);
> +		hugetlb_vma_unlock_read(vma);
> +		/* hmm_vma_fault() can retake the vma lock */
> +		ret = hmm_vma_fault(addr, end, required_fault, walk);
> +		hugetlb_vma_lock_read(vma);
> +		return ret;
>  	}
>  
>  	pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
> -- 
> 2.37.3
>