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Date: Fri, 22 May 2015 13:18:22 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, qemu-devel@nongnu.org,
        kvm@vger.kernel.org, linux-api@vger.kernel.org,
        Pavel Emelyanov <xemul@parallels.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andres Lagar-Cavilla <andreslc@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
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        Mel Gorman <mgorman@suse.de>, Andy Lutomirski <luto@amacapital.net>,
        Hugh Dickins <hughd@google.com>, Peter Feiner <pfeiner@google.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
Subject: Re: [PATCH 22/23] userfaultfd: avoid mmap_sem read recursion in
 mcopy_atomic
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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);
> +