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SubjectRe: [PATCH] scatterlist: Comment on pages for sg_set_page()
On Wed, Oct 2, 2019 at 7:27 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Oct 01, 2019 at 11:32:50AM -0700, Alan Mikhak wrote:
> > From: Alan Mikhak <alan.mikhak@sifive.com>
> >
> > Update the description of sg_set_page() to communicate current
> > requirements for the page pointer parameter.
> >
> > Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
> > include/linux/scatterlist.h | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> > index 6eec50fb36c8..6dda865893aa 100644
> > +++ b/include/linux/scatterlist.h
> > @@ -112,6 +112,12 @@ static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
> > * of the page pointer. See sg_page() for looking up the page belonging
> > * to an sg entry.
> > *
> > + * Scatterlist currently expects the page parameter to be a pointer to
> > + * a page that is backed by a page struct.
> > + *
> > + * Page pointers derived from addresses obtained from ioremap() are
> > + * currently not supported since they require use of iomem safe memcpy.
> > + *
> > **/
> > static inline void sg_set_page(struct scatterlist *sg, struct page *page,
> > unsigned int len, unsigned int offset)
>
> It seems a bit weird to have a comment explaining that 'struct page
> *page' must actually be a valid pointer. Of course it must.
>
> Computing a 'struct page *' to something that doesn't actually have a
> struct page is simply a bug in whoever did that.
>
> Code should never be interchanging ioremap results with the struct
> page* world.
>
> Jason

Agreed. Should scatterlist API sg_set_page() check the page pointer to
prevent such usage that could crash the system? Is it still reasonable
to at least tell the programmer what is not allowed by adding a
comment in the description?

Alan

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