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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 3 Feb 2017 10:29:23 -0800
Message-ID: <CA+55aFx=NPESJv9RjCNRKFH_rk9uVMov0UtFbpZH-xBsgK2h-w@mail.gmail.com>
Subject: Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to
 allocate more pages per call
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jeff Layton <jlayton@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        ceph-devel <ceph-devel@vger.kernel.org>, lustre-devel@lists.lustre.org,
        V9FS Developers <v9fs-developer@lists.sourceforge.net>,
        Jan Kara <jack@suse.cz>, Chris Wilson <chris@chris-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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Sender: linux-nfs-owner@vger.kernel.org

On Thu, Feb 2, 2017 at 11:29 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> get_user_pages() relies upon find_extend_vma() to reject kernel
> addresses; the fast side of get_user_pages_fast() doesn't have anything
> of that sort

It is *supposed* to have it.

See pte_allows_gup(), for example. In particular, it requires the
_PAGE_USER bit in the PTE (and the devpte case should require
_PAGE_BIT_DEVMAP).

So no, get_user_pages_fast() can not look up kernel pages. If it does,
that would be a bug.

              Linus