linux-kernel - Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages

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Date:   Fri, 3 Feb 2017 10:29:23 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jeff Layton <jlayton@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        lustre-devel@...ts.lustre.org,
        V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
        Jan Kara <jack@...e.cz>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to
 allocate more pages per call

On Thu, Feb 2, 2017 at 11:29 PM, Al Viro <viro@...iv.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