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    SubjectRe: [PATCH] devmem: handle partial kmem write/read
    Hi Kame,

    This patch needs more work. I first intent to fix a bug:

    sz = vwrite(kbuf, (char *)p, sz);
    p += sz;
    }

    So if the returned len is 0, the kbuf/p pointers will mismatch.

    Then I realize it changed the write behavior. The current vwrite()
    behavior is strange, it returns 0 if _whole range_ is hole, otherwise
    ignores the hole silently. So holes can be treated differently even in
    the original code.

    I'm not really sure about the right behavior. KAME-san, do you have
    any suggestions?

    Thanks,
    Fengguang

    On Mon, Sep 14, 2009 at 11:29:01AM +0800, Wu Fengguang wrote:
    > Current vwrite silently ignores vwrite() to vmap holes.
    > Better behavior should be stop the write and return
    > on such holes.
    >
    > Also return on partial read, which may happen in future
    > (eg. hit hwpoison pages).
    >
    > CC: Andi Kleen <andi@firstfloor.org>
    > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    > ---
    > drivers/char/mem.c | 30 ++++++++++++++++++------------
    > 1 file changed, 18 insertions(+), 12 deletions(-)
    >
    > --- linux-mm.orig/drivers/char/mem.c 2009-09-14 10:59:50.000000000 +0800
    > +++ linux-mm/drivers/char/mem.c 2009-09-14 11:06:25.000000000 +0800
    > @@ -444,18 +444,22 @@ static ssize_t read_kmem(struct file *fi
    > if (!kbuf)
    > return -ENOMEM;
    > while (count > 0) {
    > + unsigned long n;
    > +
    > sz = size_inside_page(p, count);
    > - sz = vread(kbuf, (char *)p, sz);
    > - if (!sz)
    > + n = vread(kbuf, (char *)p, sz);
    > + if (!n)
    > break;
    > - if (copy_to_user(buf, kbuf, sz)) {
    > + if (copy_to_user(buf, kbuf, n)) {
    > free_page((unsigned long)kbuf);
    > return -EFAULT;
    > }
    > - count -= sz;
    > - buf += sz;
    > - read += sz;
    > - p += sz;
    > + count -= n;
    > + buf += n;
    > + read += n;
    > + p += n;
    > + if (n < sz)
    > + break;
    > }
    > free_page((unsigned long)kbuf);
    > }
    > @@ -551,11 +555,13 @@ static ssize_t write_kmem(struct file *
    > free_page((unsigned long)kbuf);
    > return -EFAULT;
    > }
    > - sz = vwrite(kbuf, (char *)p, sz);
    > - count -= sz;
    > - buf += sz;
    > - virtr += sz;
    > - p += sz;
    > + n = vwrite(kbuf, (char *)p, sz);
    > + count -= n;
    > + buf += n;
    > + virtr += n;
    > + p += n;
    > + if (n < sz)
    > + break;
    > }
    > free_page((unsigned long)kbuf);
    > }


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