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SubjectRe: [PATCH] devmem: handle partial kmem write/read
On Tue, Sep 15, 2009 at 08:24:48AM +0800, KAMEZAWA Hiroyuki wrote:
> 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;

Since the 0 return value won't be used at all, it would be simpler to
tell vwrite() return the untouched buflen/sz, like this. It will ignore
_all_ the holes silently. Need to update comments too.

--- linux-mm.orig/mm/vmalloc.c 2009-09-15 09:40:08.000000000 +0800
+++ linux-mm/mm/vmalloc.c 2009-09-15 09:43:33.000000000 +0800
@@ -1834,7 +1834,6 @@ long vwrite(char *buf, char *addr, unsig
struct vm_struct *tmp;
char *vaddr;
unsigned long n, buflen;
- int copied = 0;

/* Don't allow overflow */
if ((unsigned long) addr + count < count)
@@ -1858,7 +1857,6 @@ long vwrite(char *buf, char *addr, unsig
n = count;
if (!(tmp->flags & VM_IOREMAP)) {
aligned_vwrite(buf, addr, n);
- copied++;
}
buf += n;
addr += n;
@@ -1866,8 +1864,6 @@ long vwrite(char *buf, char *addr, unsig
}
finished:
read_unlock(&vmlist_lock);
- if (!copied)
- return 0;
return buflen;
}

> ==
> 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...

I'm trying to stop possible mem/kmem users to access hwpoison pages..
I'm not the user, but rather a tester ;)

Thanks,
Fengguang

> 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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