Re: Anticipatory prefaulting in the page fault handler V1

[Akinobu Mita]

On Friday 10 December 2004 04:32, Christoph Lameter wrote:
> On Wed, 8 Dec 2004, Martin J. Bligh wrote:
> > I tried benchmarking it ... but processes just segfault all the time.
> > Any chance you could try it out on SMP ia32 system?
>
> I tried it on my i386 system and it works fine. Sorry about the puny
> memory sizes (the system is a PIII-450 with 384k memory)
>

I also encountered processes segfault.
Below patch fix several problems.

1) if no pages could allocated, returns VM_FAULT_OOM
2) fix duplicated pte_offset_map() call
3) don't set_pte() for the entry which already have been set

Acutually, 3) fixes my segfault problem.

--- 2.6-rc/mm/memory.c.orig	2004-12-13 22:17:04.000000000 +0900
+++ 2.6-rc/mm/memory.c	2004-12-13 22:22:14.000000000 +0900
@@ -1483,6 +1483,8 @@ do_anonymous_page(struct mm_struct *mm, 
 				} else
 					break;
 			}
+			if (a == addr)
+				goto no_mem;
 			end_addr = a;
 
 			spin_lock(&mm->page_table_lock);
@@ -1514,8 +1516,17 @@ do_anonymous_page(struct mm_struct *mm, 
 			}
  		} else {
  			/* Read */
+			int first = 1;
+
  			for(;addr < end_addr; addr += PAGE_SIZE) {
-				page_table = pte_offset_map(pmd, addr);
+				if (!first)
+					page_table = pte_offset_map(pmd, addr);
+				first = 0;
+				if (!pte_none(*page_table)) {
+					/* Someone else got there first */
+					pte_unmap(page_table);
+					continue;
+				}
  				entry = pte_wrprotect(mk_pte(ZERO_PAGE(addr), vma->vm_page_prot));
 				set_pte(page_table, entry);
 				pte_unmap(page_table);


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