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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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