do_wp_page: clarify dirty_page handling

Reorganize the code so that dirty pages are handled closer to the place
that makes them dirty (handling write fault into shared, writable VMAs).
No behavior changes.

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Theodore Tso <tytso@google.com>
Cc: Michael Rubin <mrubin@google.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 34 deletions

diff --git a/mm/memory.c b/mm/memory.c
index d0cc1c134a64..9144fae9a68b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2229,8 +2229,45 @@ reuse:
                 entry = maybe_mkwrite(pte_mkdirty(entry), vma);
                 if (ptep_set_access_flags(vma, address, page_table, entry,1))
                         update_mmu_cache(vma, address, page_table);
+                pte_unmap_unlock(page_table, ptl);
                 ret |= VM_FAULT_WRITE;
-                goto unlock;
+
+                if (!dirty_page)
+                        return ret;
+
+                /*
+                 * Yes, Virginia, this is actually required to prevent a race
+                 * with clear_page_dirty_for_io() from clearing the page dirty
+                 * bit after it clear all dirty ptes, but before a racing
+                 * do_wp_page installs a dirty pte.
+                 *
+                 * do_no_page is protected similarly.
+                 */
+                if (!page_mkwrite) {
+                        wait_on_page_locked(dirty_page);
+                        set_page_dirty_balance(dirty_page, page_mkwrite);
+                }
+                put_page(dirty_page);
+                if (page_mkwrite) {
+                        struct address_space *mapping = dirty_page->mapping;
+
+                        set_page_dirty(dirty_page);
+                        unlock_page(dirty_page);
+                        page_cache_release(dirty_page);
+                        if (mapping)    {
+                                /*
+                                 * Some device drivers do not set page.mapping
+                                 * but still dirty their pages
+                                 */
+                                balance_dirty_pages_ratelimited(mapping);
+                        }
+                }
+
+                /* file_update_time outside page_lock */
+                if (vma->vm_file)
+                        file_update_time(vma->vm_file);
+
+                return ret;
         }
 
         /*
@@ -2336,39 +2373,6 @@ gotten:
                 page_cache_release(old_page);
 unlock:
         pte_unmap_unlock(page_table, ptl);
-        if (dirty_page) {
-                /*
-                 * Yes, Virginia, this is actually required to prevent a race
-                 * with clear_page_dirty_for_io() from clearing the page dirty
-                 * bit after it clear all dirty ptes, but before a racing
-                 * do_wp_page installs a dirty pte.
-                 *
-                 * do_no_page is protected similarly.
-                 */
-                if (!page_mkwrite) {
-                        wait_on_page_locked(dirty_page);
-                        set_page_dirty_balance(dirty_page, page_mkwrite);
-                }
-                put_page(dirty_page);
-                if (page_mkwrite) {
-                        struct address_space *mapping = dirty_page->mapping;
-
-                        set_page_dirty(dirty_page);
-                        unlock_page(dirty_page);
-                        page_cache_release(dirty_page);
-                        if (mapping)    {
-                                /*
-                                 * Some device drivers do not set page.mapping
-                                 * but still dirty their pages
-                                 */
-                                balance_dirty_pages_ratelimited(mapping);
-                        }
-                }
-
-                /* file_update_time outside page_lock */
-                if (vma->vm_file)
-                        file_update_time(vma->vm_file);
-        }
         return ret;
 oom_free_new:
         page_cache_release(new_page);