Re: rnfs: rq_respages pointer is bad

David J. Wilder
2010-03-03T16:20:54Z

On Mon, 2010-03-01 at 21:35 -0600, Tom Tucker wrote:
> Hi David:
>
> That looks like a bug to me and it looks like what you propose is the
> correct fix. My only reservation is that if you are correct then how did
> this work at all without data corruption for large writes on x86_64?

The size of the page array is determined by page size (and some other
parameters). I am using 64k pages, that may be the key. I don't know
what page size was used on X86-64 but we may have been lucky and not
fallen off the end of the array thus avoided the problem.

>
> I'm on the road right now, so I can't dig too deep until Wednesday, but
> at this point your analysis looks correct to me.
>
> Tom
>
>
> David J. Wilder wrote:
> > Tom
> >
> > I have been chasing an rnfs related Oops in svc_process(). I have found
> > the source of the Oops but I am not sure of my fix. I am seeing the
> > problem on ppc64, kernel 2.6.32, I have not tried other arch yet.
> >
> > The source of the problem is in rdma_read_complete(), I am finding that
> > rqstp->rq_respages is set to point past the end of the rqstp->rq_pages
> > page list. This results in a NULL reference in svc_process() when
> > passing rq_respages[0] to page_address().
> >
> > In rdma_read_complete() we are using rqstp->rq_arg.pages as the base of
> > the page list then indexing by page_no, however rq_arg.pages is not
> > pointing to the start of the list so rq_respages ends up pointing to:
> >
> > rqstp->rq_pages[(head->count+1) + head->hdr_count]
> >
> > In my case, it ends up pointing one past the end of the list by one.
> >
> > Here is the change I made.
> >
> > static int rdma_read_complete(struct svc_rqst *rqstp,
> > struct svc_rdma_op_ctxt *head)
> > {
> > int page_no;
> > int ret;
> >
> > BUG_ON(!head);
> >
> > /* Copy RPC pages */
> > for (page_no = 0; page_no < head->count; page_no++) {
> > put_page(rqstp->rq_pages[page_no]);
> > rqstp->rq_pages[page_no] = head->pages[page_no];
> > }
> > /* Point rq_arg.pages past header */
> > rqstp->rq_arg.pages = &rqstp->rq_pages[head->hdr_count];
> > rqstp->rq_arg.page_len = head->arg.page_len;
> > rqstp->rq_arg.page_base = head->arg.page_base;
> >
> > /* rq_respages starts after the last arg page */
> > - rqstp->rq_respages = &rqstp->rq_arg.pages[page_no];
> > + rqstp->rq_respages = &rqstp->rq_pages[page_no];
> > .
> > .
> > .
> >
> > The change works for me, but I am not sure it is safe to assume the
> > rqstp->rq_pages[head->count] will always point to the last arg page.
> >
> > Dave.
> >
>
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