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    SubjectRe: [PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare
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    On 12/7/22 12:30, 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.
    >
    > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
    > Signed-off-by: Peter Xu <peterx@redhat.com>
    > ---
    > arch/s390/mm/gmap.c | 2 ++
    > fs/proc/task_mmu.c | 2 ++
    > include/linux/pagewalk.h | 11 ++++++++++-
    > mm/hmm.c | 15 ++++++++++++++-
    > mm/pagewalk.c | 2 ++
    > 5 files changed, 30 insertions(+), 2 deletions(-)

    Reviewed-by: John Hubbard <jhubbard@nvidia.com>


    thanks,
    --
    John Hubbard
    NVIDIA

    >
    > 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 e35a0398db63..cf3887fb2905 100644
    > --- a/fs/proc/task_mmu.c
    > +++ b/fs/proc/task_mmu.c
    > @@ -1613,7 +1613,9 @@ 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..27a6df448ee5 100644
    > --- a/include/linux/pagewalk.h
    > +++ b/include/linux/pagewalk.h
    > @@ -21,7 +21,16 @@ 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. This hook
    > + * function is called with the vma lock held, in order to
    > + * protect against a concurrent freeing of the pte_t* or
    > + * the ptl. In some cases, the hook function needs to drop
    > + * and retake the vma lock in order to avoid deadlocks
    > + * while calling other functions. In such cases the hook
    > + * function must either refrain from accessing the pte or
    > + * ptl after dropping the vma lock, or else revalidate
    > + * those items after re-acquiring the vma lock and before
    > + * accessing them.
    > * @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..796de6866089 100644
    > --- a/mm/hmm.c
    > +++ b/mm/hmm.c
    > @@ -493,8 +493,21 @@ 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);
    > + /*
    > + * Avoid deadlock: drop the vma lock before calling
    > + * hmm_vma_fault(), which will itself potentially take and
    > + * drop the vma lock. This is also correct from a
    > + * protection point of view, because there is no further
    > + * use here of either pte or ptl after dropping 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);
    > 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);
    > @@ -314,6 +315,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
    > if (err)
    > break;
    > } while (addr = next, addr != end);
    > + hugetlb_vma_unlock_read(vma);
    >
    > return err;
    > }

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