From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 8 Jan 2003 10:05:53 +1100 From: David Gibson To: devel list Subject: Re: get_pteptr mystery Message-ID: <20030107230553.GA7851@zax.zax> References: <1040256384.30597.70.camel@granite.austin.ibm.com> <1041896173.1207.49.camel@granite.austin.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1041896173.1207.49.camel@granite.austin.ibm.com> Sender: owner-linuxppc-dev@lists.linuxppc.org List-Id: On Mon, Jan 06, 2003 at 05:35:58PM -0600, Hollis Blanchard wrote: > > On Wed, 2002-12-18 at 18:06, Hollis Blanchard wrote: > > > > I need to mark the first kernel code page writeable because I need to > > write to 0xc00000fc. Can someone tell me why the following code fails? > > get_pteptr returns 0. > > > > addr = 0xc00000fc; > > if (0 != get_pteptr(&init_mm, addr, &ptep)) { > > /* mark it writable */ > > *ptep = pte_mkwrite(*ptep); > > /* flush this page from hw TLB */ > > flush_tlb_range(&init_mm, addr, addr+1); > > } else { > > printk(KERN_ERR "couldn't get PTE!\n"); > > } > > FYI: whoever added HIGHMEM support to mm/init.c:paging_init() already > had this problem, and solved it by calling map_page(addr, 0, 0) first. > > I guess that the page is really not present in the software pagetable, > and the map_page call forces it in: > map_page(PKMAP_BASE, 0, 0); /* XXX gross */ > pkmap_page_table = pte_offset(pmd_offset(pgd_offset_k(PKMAP_BASE), > PKMAP_BASE), PKMAP_BASE); > (get_pteptr just calls pte/pmd/pgd_offset with some error checking.) Erm.. be very careful about this in 2.5. Since we use a large-page mapping for that address there, it won't in fact have a "normal" PTE. So attempting to frob it could cause bad things to happen. -- David Gibson | For every complex problem there is a david@gibson.dropbear.id.au | solution which is simple, neat and | wrong. http://www.ozlabs.org/people/dgibson ** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/