[PATCH] more ZERO_PAGE handling ( was 2.6.24 regression: deadlockon coredump of big process)

[KAMEZAWA Hiroyuki]

On Tue, 29 Apr 2008 10:10:58 -0400
Tony Battersby <tonyb@xxxxxxxxxxxxxxx> wrote:
> 
> If I leave more memory free by changing the argument to
> malloc_all_but_x_mb(), then I have to increase the number of threads
> required to trigger the deadlock.  Changing the thread stack size via
> setrlimit(RLIMIT_STACK) also changes the number of threads that are
> required to trigger the deadlock.  For example, with
> malloc_all_but_x_mb(16) and the default stack size of 8 MB, <= 5 threads
> will coredump successfully, and >= 6 threads will deadlock.  With
> malloc_all_but_x_mb(16) and a reduced stack size of 4096 bytes, <= 8
> threads will coredump successfully, and >= 9 threads will deadlock.
> 
> Also note that the "free" command reports 10 MB free memory while the
> program is running before the segfault is triggered.
> 
Hmm, my idea is below.

Nick's remove ZERO_PAGE patch includes following change

==
@@ -2252,39 +2158,24 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
        spinlock_t *ptl;
 {
<snip>
-               page_add_new_anon_rmap(page, vma, address);
-       } else {
-               /* Map the ZERO_PAGE - vm_page_prot is readonly */
-               page = ZERO_PAGE(address);
-               page_cache_get(page);
-               entry = mk_pte(page, vma->vm_page_prot);
+       if (unlikely(anon_vma_prepare(vma)))
+               goto oom;
+       page = alloc_zeroed_user_highpage_movable(vma, address);
==

above change is for avoiding to use ZERO_PAGE at read-page-fault to anonymous
vma. This is reasonable I think. But at coredump, tons of read-but-never-written 
pages can be allocated.
==
coredump
  -> get_user_pages()
       -> follow_page() returns NULL
            -> handle mm fault
                 -> do_anonymous page.
==
follow_page() returns ZERO_PAGE only when page table is not avaiable.

So, making follow_page() return ZERO_PAGE can be a fix of extra memory
consumpstion at core dump. (Maybe someone can think of other fix.)

how about this patch ? Could you try ?

(I'm sorry but I'll not be active for a week because my servers are powered off.)

-Kame

==
follow_page() returns ZERO_PAGE if page table is not available.
but returns NULL pte is not presentl.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Index: linux-2.6.25/mm/memory.c
===================================================================
--- linux-2.6.25.orig/mm/memory.c
+++ linux-2.6.25/mm/memory.c
@@ -926,15 +926,15 @@ struct page *follow_page(struct vm_area_
 	page = NULL;
 	pgd = pgd_offset(mm, address);
 	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
-		goto no_page_table;
+		goto null_or_zeropage;
 
 	pud = pud_offset(pgd, address);
 	if (pud_none(*pud) || unlikely(pud_bad(*pud)))
-		goto no_page_table;
+		goto null_or_zeropage;
 	
 	pmd = pmd_offset(pud, address);
 	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
-		goto no_page_table;
+		goto null_or_zeropage;
 
 	if (pmd_huge(*pmd)) {
 		BUG_ON(flags & FOLL_GET);
@@ -947,8 +947,10 @@ struct page *follow_page(struct vm_area_
 		goto out;
 
 	pte = *ptep;
-	if (!pte_present(pte))
-		goto unlock;
+	if (!(flags & FOLL_WRITE) && !pte_present(pte)) {
+		pte_unmap_unlock(ptep, ptl);
+		goto null_or_zeropage;
+	}
 	if ((flags & FOLL_WRITE) && !pte_write(pte))
 		goto unlock;
 	page = vm_normal_page(vma, address, pte);
@@ -968,7 +970,7 @@ unlock:
 out:
 	return page;
 
-no_page_table:
+null_or_zeropage:
 	/*
 	 * When core dumping an enormous anonymous area that nobody
 	 * has touched so far, we don't want to allocate page tables.

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