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

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Date:	Wed, 30 Apr 2008 14:35:42 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Mika Penttilä <mika.penttila@...umbus.fi>,
	Tony Battersby <tonyb@...ernetics.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] more ZERO_PAGE handling ( was 2.6.24 regression:
 deadlock on coredump of big process)

On Wed, 30 Apr 2008 07:19:32 +0200
Nick Piggin <npiggin@...e.de> wrote:

> 
> Something like this should do:
> if (!pte_present(pte)) {
> 	if (pte_none(pte)) {
> 		pte_unmap_unlock
> 		goto null_or_zeropage;
> 	}
> 	goto unlock;
> }
> 
Sorry for broken work and thank you for advice.
updated.

Regards,
-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_present()/pte_none() in proper way.


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

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 +947,13 @@ struct page *follow_page(struct vm_area_
 		goto out;
 
 	pte = *ptep;
-	if (!pte_present(pte))
+	if (!pte_present(pte)) {
+		if (!(flags & FOLL_WRITE) && pte_none(pte)) {
+			pte_unmap_unlock(ptep, ptl);
+			goto null_or_zeropage;
+		}
 		goto unlock;
+	}
 	if ((flags & FOLL_WRITE) && !pte_write(pte))
 		goto unlock;
 	page = vm_normal_page(vma, address, pte);
@@ -968,7 +973,7 @@ 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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