Commit 8e4b9a60 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files


The "FOLL_ANON optimization" and its use_zero_page() test have caused
confusion and bugs: why does it test VM_SHARED? for the very good but
unsatisfying reason that VMware crashed without.  As we look to maybe
reinstating anonymous use of the ZERO_PAGE, we need to sort this out.

Easily done: it's silly for __get_user_pages() and follow_page() to
be guessing whether it's safe to assume that they're being used for
a coredump (which can take a shortcut snapshot where other uses must
handle a fault) - just tell them with GUP_FLAGS_DUMP and FOLL_DUMP.

get_dump_page() doesn't even want a ZERO_PAGE: an error suits fine.
Signed-off-by: default avatarHugh Dickins <>
Acked-by: default avatarRik van Riel <>
Acked-by: default avatarMel Gorman <>
Reviewed-by: default avatarMinchan Kim <>
Cc: KAMEZAWA Hiroyuki <>
Cc: KOSAKI Motohiro <>
Cc: Nick Piggin <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent f3e8fccd
......@@ -1231,7 +1231,7 @@ struct page *follow_page(struct vm_area_struct *, unsigned long address,
#define FOLL_WRITE 0x01 /* check pte is writable */
#define FOLL_TOUCH 0x02 /* mark page accessed */
#define FOLL_GET 0x04 /* do get_page on page */
#define FOLL_ANON 0x08 /* give ZERO_PAGE if no pgtable */
#define FOLL_DUMP 0x08 /* give error on hole if it would be zero */
typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
......@@ -252,6 +252,7 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn,
#define GUP_FLAGS_WRITE 0x01
#define GUP_FLAGS_FORCE 0x02
#define GUP_FLAGS_DUMP 0x04
int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
unsigned long start, int len, int flags,
......@@ -1174,41 +1174,22 @@ no_page:
pte_unmap_unlock(ptep, ptl);
if (!pte_none(pte))
return page;
/* Fall through to ZERO_PAGE handling */
* When core dumping an enormous anonymous area that nobody
* has touched so far, we don't want to allocate page tables.
* has touched so far, we don't want to allocate unnecessary pages or
* page tables. Return error instead of NULL to skip handle_mm_fault,
* then get_dump_page() will return NULL to leave a hole in the dump.
* But we can only make this optimization where a hole would surely
* be zero-filled if handle_mm_fault() actually did handle it.
if (flags & FOLL_ANON) {
page = ZERO_PAGE(0);
if (flags & FOLL_GET)
if ((flags & FOLL_DUMP) &&
(!vma->vm_ops || !vma->vm_ops->fault))
return ERR_PTR(-EFAULT);
return page;
/* Can we do the FOLL_ANON optimization? */
static inline int use_zero_page(struct vm_area_struct *vma)
* We don't want to optimize FOLL_ANON for make_pages_present()
* when it tries to page in a VM_LOCKED region. As to VM_SHARED,
* we want to get the page from the page tables to make sure
* that we serialize and update with any other user of that
* mapping.
if (vma->vm_flags & (VM_LOCKED | VM_SHARED))
return 0;
* And if we have a fault routine, it's not an anonymous region.
return !vma->vm_ops || !vma->vm_ops->fault;
int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
unsigned long start, int nr_pages, int flags,
struct page **pages, struct vm_area_struct **vmas)
......@@ -1288,8 +1269,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
foll_flags = FOLL_TOUCH;
if (pages)
foll_flags |= FOLL_GET;
if (!write && use_zero_page(vma))
foll_flags |= FOLL_ANON;
if (flags & GUP_FLAGS_DUMP)
foll_flags |= FOLL_DUMP;
do {
struct page *page;
......@@ -1446,7 +1427,7 @@ struct page *get_dump_page(unsigned long addr)
struct page *page;
if (__get_user_pages(current, current->mm, addr, 1,
GUP_FLAGS_FORCE, &page, &vma) < 1)
GUP_FLAGS_FORCE | GUP_FLAGS_DUMP, &page, &vma) < 1)
return NULL;
if (page == ZERO_PAGE(0)) {
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment