Commit dd151b39 authored by Andrew Morton's avatar Andrew Morton Committed by Linus Torvalds

[PATCH] get_user_pages shortcut for anonymous pages

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

The patch avoids the instantiation of pagetables for not-present pages in
get_user_pages().  Without this, the coredump code can cause total memory
exhaustion in pagetables.  Consider a store to current stack - 1TB.  The
stack vma is extended to include this address because of VM_GROWSDOWN.  If
such a process dies (which is likely for a defunc process) then the elf core
dumper will cause the system to hang because of too many page tables.

We especially recognise this situation and simply return a ref to the zero
page.
parent a3f84eac
......@@ -688,6 +688,32 @@ static inline struct page *get_page_map(struct page *page)
}
static inline int
untouched_anonymous_page(struct mm_struct* mm, struct vm_area_struct *vma,
unsigned long address)
{
pgd_t *pgd;
pmd_t *pmd;
/* Check if the vma is for an anonymous mapping. */
if (vma->vm_ops && vma->vm_ops->nopage)
return 0;
/* Check if page directory entry exists. */
pgd = pgd_offset(mm, address);
if (pgd_none(*pgd) || pgd_bad(*pgd))
return 1;
/* Check if page middle directory entry exists. */
pmd = pmd_offset(pgd, address);
if (pmd_none(*pmd) || pmd_bad(*pmd))
return 1;
/* There is a pte slot for 'address' in 'mm'. */
return 0;
}
int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
unsigned long start, int len, int write, int force,
struct page **pages, struct vm_area_struct **vmas)
......@@ -750,6 +776,18 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
struct page *map;
int lookup_write = write;
while (!(map = follow_page(mm, start, lookup_write))) {
/*
* Shortcut for anonymous pages. We don't want
* to force the creation of pages tables for
* insanly big anonymously mapped areas that
* nobody touched so far. This is important
* for doing a core dump for these mappings.
*/
if (!lookup_write &&
untouched_anonymous_page(mm,vma,start)) {
map = ZERO_PAGE(start);
break;
}
spin_unlock(&mm->page_table_lock);
switch (handle_mm_fault(mm,vma,start,write)) {
case VM_FAULT_MINOR:
......
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