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SubjectRe: [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@suse.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@jp.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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