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SubjectRe: [RFC PATCH 2/3] hugetlbfs: introduce hinode_rwsem for pmd sharing synchronization
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On Tue, Oct 13, 2020 at 04:10:59PM -0700, Mike Kravetz wrote:
> Due to pmd sharing, the huge PTE pointer returned by huge_pte_alloc
> may not be valid. This can happen if a call to huge_pmd_unshare for
> the same pmd is made in another thread.
>
> To address this issue, add a rw_semaphore (hinode_rwsem) to the hugetlbfs
> inode.
> - hinode_rwsem is taken in read mode before calling huge_pte_alloc, and
> held until finished with the returned pte pointer.
> - hinode_rwsem is held in write mode whenever huge_pmd_unshare is called.
>
> In the locking hierarchy, hinode_rwsem must be taken before a page lock.
>
> In an effort to minimize performance impacts, hinode_rwsem is not taken
> if the caller knows the target can not possibly be part of a shared pmd.
> lockdep_assert calls are added to huge_pmd_share and huge_pmd_unshare to
> help catch callers not using the proper locking.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Hi Mike,

I didn't find a problem on main idea of introducing hinode_rwsem, so
I'm fine if the known problems are fixed.

I have one question. This patch seems to make sure that huge_pmd_unshare()
are called under holding hinode_rwsem in write mode for some case. Some
callers of try_to_unmap() seem not to hold it like shrink_page_list(),
unmap_page(), which is OK because they never call try_to_unmap() for hugetlb
pages. And unmap_ref_private() doesn't takes hinode_rwsem either, and
that's also OK because this function never handles pmd sharing case. So
what about unmap_single_vma()? It seems that this generic function could
reach huge_pmd_unshare() without hinode_rwsem, so what prevents the race here?
(Maybe I might miss some assumption or condition over this race...)

I left a few other minor comments below ...

> @@ -4424,6 +4442,11 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>
> ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> if (ptep) {
> + /*
> + * Since we hold no locks, ptep could be stale. That is
> + * OK as we are only making decisions based on content and
> + * not actually modifying content here.
> + */

nice comment, thank you.

> entry = huge_ptep_get(ptep);
> if (unlikely(is_hugetlb_entry_migration(entry))) {
> migration_entry_wait_huge(vma, mm, ptep);
> @@ -4431,20 +4454,32 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> return VM_FAULT_HWPOISON_LARGE |
> VM_FAULT_SET_HINDEX(hstate_index(h));
> - } else {
> - ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
> - if (!ptep)
> - return VM_FAULT_OOM;
> }
>
> + /*
> + * Acquire hinode_sem before calling huge_pte_alloc and hold

hinode_rwsem?

> + * until finished with ptep. This prevents huge_pmd_unshare from
> + * being called elsewhere and making the ptep no longer valid.
> + *
> + * ptep could have already be assigned via huge_pte_offset. That
> + * is OK, as huge_pte_alloc will return the same value unless
> + * something has changed.
> + */

...

> @@ -278,10 +278,14 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> BUG_ON(dst_addr >= dst_start + len);
>
> /*
> - * Serialize via hugetlb_fault_mutex
> + * Serialize via hinode_rwsem hugetlb_fault_mutex.
^ "and" here?

> + * hinode_rwsem ensures the dst_pte remains valid even
> + * in the case of shared pmds. fault mutex prevents
> + * races with other faulting threads.
> */
> idx = linear_page_index(dst_vma, dst_addr);
> mapping = dst_vma->vm_file->f_mapping;
> + hinode_lock_read(mapping, dst_vma, dst_addr);
> hash = hugetlb_fault_mutex_hash(mapping, idx);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);


Thanks,
Naoya Horiguchi
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