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    SubjectRe: [PATCH 22/23] userfaultfd: avoid mmap_sem read recursion in mcopy_atomic
    On Thu, 14 May 2015 19:31:19 +0200 Andrea Arcangeli <aarcange@redhat.com> wrote:

    > If the rwsem starves writers it wasn't strictly a bug but lockdep
    > doesn't like it and this avoids depending on lowlevel implementation
    > details of the lock.
    >
    > ...
    >
    > @@ -229,13 +246,33 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
    >
    > if (!zeropage)
    > err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
    > - dst_addr, src_addr);
    > + dst_addr, src_addr, &page);
    > else
    > err = mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma,
    > dst_addr);
    >
    > cond_resched();
    >
    > + if (unlikely(err == -EFAULT)) {
    > + void *page_kaddr;
    > +
    > + BUILD_BUG_ON(zeropage);

    I'm not sure what this is trying to do. BUILD_BUG_ON(local_variable)?

    It goes bang in my build. I'll just delete it.

    > + up_read(&dst_mm->mmap_sem);
    > + BUG_ON(!page);
    > +
    > + page_kaddr = kmap(page);
    > + err = copy_from_user(page_kaddr,
    > + (const void __user *) src_addr,
    > + PAGE_SIZE);
    > + kunmap(page);
    > + if (unlikely(err)) {
    > + err = -EFAULT;
    > + goto out;
    > + }
    > + goto retry;
    > + } else
    > + BUG_ON(page);
    > +



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