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

[KAMEZAWA Hiroyuki]

On Wed, 30 Apr 2008 08:03:33 +0300
Mika Penttilà <mika.penttila@xxxxxxxxxxx> wrote:

> > ==
> > @@ -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
> >
> >   
> 
> 
> But sure we still have to handle the fault for instance swapped pages, 
> for other uses of get_user_pages();
> 
Ah, my bad.....how about this ? I changed !pte_present() to pte_none().

-Kame
==
follow_page() returns ZERO_PAGE if a page table is not available.
but returns NULL if a page table exists. If NULL, handle_mm_fault()
allocates a new page.

This behavior increases page consumption at coredump, which tend
to do read-once-but-never-written page fault.  This patch is
for avoiding this.

Changelog:
  - fixed to check pte_none() not !pte_present().


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_none(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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