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    SubjectRe: [patch] ksm: check for ERR_PTR from follow_page()
    On Wed, Apr 21, 2010 at 07:46:15PM +0200, Andrea Arcangeli wrote:
    > On Wed, Apr 21, 2010 at 11:24:24AM -0400, Rik van Riel wrote:
    > > On 04/21/2010 06:27 AM, Dan Carpenter wrote:
    > > > The follow_page() function can potentially return -EFAULT so I added
    > > > checks for this.
    > > >
    > > > Also I silenced an uninitialized variable warning on my version of gcc
    > > > (version 4.3.2).
    > > >
    > > > Signed-off-by: Dan Carpenter<error27@gmail.com>
    > >
    > > Acked-by: Rik van Riel <riel@redhat.com>
    >
    > while (!(page = follow_page(vma, start, foll_flags)))
    > {
    >
    > gup only checks for null, so when exactly is follow_page going to
    > return -EFAULT? It's not immediately clear.

    Check below that loop. If it returns non-null, the first check is
    whether it IS_ERR().

    How about the below?

    Hannes

    ---
    From: Johannes Weiner <hannes@cmpxchg.org>
    Subject: mm: document follow_page()

    Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
    ---
    mm/memory.c | 13 +++++++++++--
    1 files changed, 11 insertions(+), 2 deletions(-)

    diff --git a/mm/memory.c b/mm/memory.c
    index 833952d..119b7cc 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -1227,8 +1227,17 @@ int zap_vma_ptes(struct vm_area_struct *vma, unsigned long address,
    }
    EXPORT_SYMBOL_GPL(zap_vma_ptes);

    -/*
    - * Do a quick page-table lookup for a single page.
    +/**
    + * follow_page - look up a page descriptor from a user-virtual address
    + * @vma: vm_area_struct mapping @address
    + * @address: virtual address to look up
    + * @flags: flags modifying lookup behaviour
    + *
    + * @flags can have FOLL_ flags set, defined in <linux/mm.h>
    + *
    + * Returns the mapped (struct page *), %NULL if no mapping exists, or
    + * an error pointer if there is a mapping to something not represented
    + * by a page descriptor (see also vm_normal_page()).
    */
    struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
    unsigned int flags)
    --
    1.7.0.2

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