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Date:   Mon, 19 Jul 2021 16:12:44 -0400
From:   Peter Xu <peterx@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Alistair Popple <apopple@nvidia.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Tiberiu Georgescu <tiberiu.georgescu@nutanix.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Miaohe Lin <linmiaohe@huawei.com>
Subject: Re: [PATCH v5 00/26] userfaultfd-wp: Support shmem and hugetlbfs
Message-ID: <YPXcvJ6TEzALyb/v@t490s>
References: <20210715201422.211004-1-peterx@redhat.com>
 <251ed5e3-d898-efdc-ca5c-7b047dc80cb4@redhat.com>
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On Mon, Jul 19, 2021 at 09:21:18PM +0200, David Hildenbrand wrote:
> Hi Peter,

Hi, David,

> 
> I just stumbled over copy_page_range() optimization
> 
> 	/*
> 	 * Don't copy ptes where a page fault will fill them correctly.
> 	 * Fork becomes much lighter when there are big shared or private
> 	 * readonly mappings. The tradeoff is that copy_page_range is more
> 	 * efficient than faulting.
> 	 */
> 	if (!(src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP)) &&
> 	    !src_vma->anon_vma)
> 		return 0;
> 
> IIUC, that means you'll not copy the WP bits for shmem and,
> therefore, lose them during fork.

Good point.

I think the fix shouldn't be hard - we can also skip this if dst_vma->vm_flags
has VM_UFFD_WP set (that means UFFD_FEATURE_EVENT_FORK is enabled too).  But
I'll check a bit into page copy later to make sure it works (maybe I'll add a
small test case too).

Thanks!

-- 
Peter Xu