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    SubjectRe: [PATCH] devmem: handle partial kmem write/read
    On Mon, 14 Sep 2009 12:34:44 +0800
    Wu Fengguang <fengguang.wu@intel.com> wrote:

    > 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.
    >
    Ah, ok...

    > I'm not really sure about the right behavior. KAME-san, do you have
    > any suggestions?
    >
    maybe following make sense.
    =
    written = vwrite(kbuf, (char *p), sz);
    if (!written) // whole vmem was a hole
    written = sz;
    ==
    needs fix.
    Anyway, I wonder kmem is broken. It's should be totally rewritten.

    For example, this doesn't check anything.
    ==
    if (p < (unsigned long) high_memory) {
    ==

    But....are there users ?
    If necessary, I'll write some...
    Thanks,
    -Kame


    > 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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