Re: Anticipatory prefaulting in the page fault handler V1

[Akinobu Mita]

On Tuesday 14 December 2004 21:24, Akinobu Mita wrote:

> But there is not any guarantee that the page_tables for addr+PAGE_SIZE,
> addr+2*PAGE_SIZE, ...  have not been mapped yet.
>
> Anyway, I will try your V2 patch.
>

Below patch fixes V2 patch, and adds debug printk. 
The output coincides with segfaulted processes.

# dmesg | grep ^comm:

comm: xscreensaver, addr_orig: ccdc40, addr: cce000, pid: 2995
comm: rhn-applet-gui, addr_orig: b6fd8020, addr: b6fd9000, pid: 3029
comm: rhn-applet-gui, addr_orig: b6e95020, addr: b6e96000, pid: 3029
comm: rhn-applet-gui, addr_orig: b6fd8020, addr: b6fd9000, pid: 3029
comm: rhn-applet-gui, addr_orig: b6e95020, addr: b6e96000, pid: 3029
comm: rhn-applet-gui, addr_orig: b6fd8020, addr: b6fd9000, pid: 3029
comm: X, addr_orig: 87e8000, addr: 87e9000, pid: 2874
comm: X, addr_orig: 87ea000, addr: 87eb000, pid: 2874

---
The read access prefaulting may override the page_table which has been
already mapped. this patch fixes it. and it shows which process might
suffer this problem.


--- 2.6-rc/mm/memory.c.orig	2004-12-14 22:06:08.000000000 +0900
+++ 2.6-rc/mm/memory.c	2004-12-14 23:42:34.000000000 +0900
@@ -1434,6 +1434,7 @@ do_anonymous_page(struct mm_struct *mm, 
 {
 	pte_t entry;
  	unsigned long end_addr;
+ 	unsigned long addr_orig = addr;
 
 	addr &= PAGE_MASK;
 
@@ -1517,9 +1518,15 @@ do_anonymous_page(struct mm_struct *mm, 
  		/* Read */
 		entry = pte_wrprotect(mk_pte(ZERO_PAGE(addr), vma->vm_page_prot));
 nextread:
-		set_pte(page_table, entry);
-		pte_unmap(page_table);
-		update_mmu_cache(vma, addr, entry);
+		if (!pte_none(*page_table)) {
+			printk("comm: %s, addr_orig: %lx, addr: %lx, pid: %d\n",
+				current->comm, addr_orig, addr, current->pid);
+			pte_unmap(page_table);
+		} else {
+			set_pte(page_table, entry);
+			pte_unmap(page_table);
+			update_mmu_cache(vma, addr, entry);
+		}
 		addr += PAGE_SIZE;
 		if (unlikely(addr < end_addr)) {
 			pte_offset_map(pmd, addr);


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