linux-kernel - Re: [PATCH v5 00/26] userfaultfd-wp: Support shmem and hugetlbfs

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Date:   Mon, 19 Jul 2021 16:12:44 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Alistair Popple <apopple@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Tiberiu Georgescu <tiberiu.georgescu@...anix.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>
Subject: Re: [PATCH v5 00/26] userfaultfd-wp: Support shmem and hugetlbfs

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