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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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