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Subject[PATCH 4/8] mm: FOLL_DUMP replace FOLL_ANON
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: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---
include/linux/mm.h | 2 +-
mm/internal.h | 1 +
mm/memory.c | 43 ++++++++++++-------------------------------
3 files changed, 14 insertions(+), 32 deletions(-)
--- mm3/include/linux/mm.h 2009-09-07 13:16:32.000000000 +0100
+++ mm4/include/linux/mm.h 2009-09-07 13:16:39.000000000 +0100
@@ -1247,7 +1247,7 @@ struct page *follow_page(struct vm_area_
#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);
--- mm3/mm/internal.h 2009-09-07 13:16:22.000000000 +0100
+++ mm4/mm/internal.h 2009-09-07 13:16:39.000000000 +0100
@@ -252,6 +252,7 @@ static inline void mminit_validate_memmo

#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,
--- mm3/mm/memory.c 2009-09-07 13:16:32.000000000 +0100
+++ mm4/mm/memory.c 2009-09-07 13:16:39.000000000 +0100
@@ -1174,41 +1174,22 @@ no_page:
pte_unmap_unlock(ptep, ptl);
if (!pte_none(pte))
return page;
- /* Fall through to ZERO_PAGE handling */
+
no_page_table:
/*
* 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)
- get_page(page);
- BUG_ON(flags & FOLL_WRITE);
- }
+ 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
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;
@@ -1447,7 +1428,7 @@ struct page *get_dump_page(unsigned long
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)) {
page_cache_release(page);

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