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

Mike Kravetz
2022-12-06T18:53:20Z

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@redhat.com>
> > > > ---
> > > > 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@oracle.com>
--
Mike Kravetz

> From 9ad1e65a31f51a0dc687cd9d6083b9e920d2da61 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@redhat.com>
> 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@redhat.com>
> ---
> 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
>