On Mon, Mar 18, 2013 at 11:30:47PM +0000, Ben Hutchings wrote:
> On Mon, 2013-03-18 at 14:00 -0700, Nicholas A. Bellinger wrote:
> > On Mon, 2013-03-18 at 13:35 +0800, Asias He wrote:
> > > On Sat, Mar 16, 2013 at 02:10:22AM +0000, Ben Hutchings wrote:
> > > > On Tue, 2013-03-12 at 15:44 -0700, Greg Kroah-Hartman wrote:
> > > > > 3.0-stable review patch. If anyone has any objections, please let me
> > > > > know.
> > > > >
> > > > > ------------------
> > > > >
> > > > > From: Asias He <as...@redhat.com>
> > > > >
> > > > > commit 472b72f2db7831d7dbe22ffdff4adee3bd49b05d upstream.
> > > > >
> > > > > The page++ is wrong. It makes bio_add_pc_page() pointing to a wrong
> > > > > page
> > > > > address if the 'while (len > 0 && data_len > 0) { ... }' loop is
> > > > > executed more than one once.
> > > > >
> > > > > Signed-off-by: Asias He <as...@redhat.com>
> > > > > Signed-off-by: Nicholas Bellinger <n...@linux-iscsi.org>
> > > > > Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> > > > >
> > > > > ---
> > > > > drivers/target/target_core_pscsi.c | 1 -
> > > > > 1 file changed, 1 deletion(-)
> > > > >
> > > > > --- a/drivers/target/target_core_pscsi.c
> > > > > +++ b/drivers/target/target_core_pscsi.c
> > > > > @@ -1210,7 +1210,6 @@ static int __pscsi_map_task_SG(
> > > > > bio = NULL;
> > > > > }
> > > > >
> > > > > - page++;
> > > > > len -= bytes;
> > > > > data_len -= bytes;
> > > > > off = 0;
> > > >
> > > > So in case a fragment crosses a page boundary, we wrap around to the
> > > > beginning of the same page? That doesn't look right.
> > >
> > > If the fragment crosses a page boundary, what is the correct page
> > > for it?
> > >
> > > Nicholas, can we assume sg->length + sg->offset should be less than
> > > PAGE_SIZE here?
> > >
> >
> > sg->length + sg->offset can be less than or equal to PAGE_SIZE here.
> >
> > For everything other than tcm_loop + tcm_vhost using externally
> > allocated SGLs, we can expect fragments to never cross the page
> > boundary.
> >
> > For tcm_loop + tcm_vhost, there are a few special cases with control CDB
> > paylaods (usually going through scsi-generic) where we can have a non
> > zero sg->offset, but at least in the cases I've seen this is still not
> > using SGL elements that exceed PAGE_SIZE.
> >
> > So, I think this logic is OK for SGLs that cross page boundries, given
> > that it's done outside of the inner loop where *page is set during each
> > for_each_sg().
>
> The page is set using sg_page() in the outer loop and was then
> incremented in the inner loop.
>
> for_each_sg(sgl, sg, sgl_nents, i) {
> page = sg_page(sg);
> off = sg->offset;
> len = sg->length;
>
> while (len > 0 && data_len > 0) {
> bytes = min_t(unsigned int, len, PAGE_SIZE - off);
> bytes = min(bytes, data_len);
> ...
> /* page++; */
> len -= bytes;
> data_len -= bytes;
> off = 0;
> }
> }
>
> The inner loop is apparently meant to iterate over pages of a segment,
> but is now just wrapping around a single page.
Yes, but we never loop more than once in the inner loop.
So, how about
1) fail on sg->offset + sg->length > PAGE_SIZE (we can not find a
proper page address in this case)
2) remove the inner while loop, run the code was in the loop only once.
> > The case where this logic is broken, and that the 'page
> > ++' was addressing is when sg->length > PAGE_SIZE.
>
> That is a sufficient but not a necessary condition for the increment.
>
> Ben.
>
> --
> Ben Hutchings
> Never attribute to conspiracy what can adequately be explained by stupidity.
--
Asias
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