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

Peter Xu
2022-12-06T16:46:18Z

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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

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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