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