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    SubjectRe: [ 04/21] target/pscsi: Fix page increment
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    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 <asias@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 <asias@redhat.com>
    > > > Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
    > > > Signed-off-by: Greg Kroah-Hartman <gregkh@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 case where this logic is broken, and that the 'page
    ++' was addressing is when sg->length > PAGE_SIZE.

    --nab



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