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>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 31 deletions

diff --git a/mm/memory.c b/mm/memory.c
index a8430ff13837..532a55bce6a4 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -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 *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)) {
                 page_cache_release(page);