ceph: use attach/detach_page_private for tracking snap context

There is some ambiguity around the use of PagePrivate. It's
generally expected in core code that if PagePrivate is set then
you have a reference to it. It's not clear that ceph always
does (and I believe it may not).

Change ceph to use attach/detach_page_private so that we keep a
reference to the page until the snap context is detached.

Link: https://lore.kernel.org/ceph-devel/2503810.1616508988@warthog.procyon.org.uk/T/#mf29e5abbb0ec8035cde0de30778690de7d956f84
Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index cb88ba9..72c0901 100644 (file)
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -128,8 +128,7 @@ static int ceph_set_page_dirty(struct page *page)
         * PagePrivate so that we get invalidatepage callback.
         */
        BUG_ON(PagePrivate(page));
-       page->private = (unsigned long)snapc;
-       SetPagePrivate(page);
+       attach_page_private(page, snapc);
 
        ret = __set_page_dirty_nobuffers(page);
        WARN_ON(!PageLocked(page));
@@ -148,7 +147,7 @@ static void ceph_invalidatepage(struct page *page, unsigned int offset,
 {
        struct inode *inode;
        struct ceph_inode_info *ci;
-       struct ceph_snap_context *snapc = page_snap_context(page);
+       struct ceph_snap_context *snapc;
 
        wait_on_page_fscache(page);
 
@@ -168,10 +167,9 @@ static void ceph_invalidatepage(struct page *page, unsigned int offset,
        dout("%p invalidatepage %p idx %lu full dirty page\n",
             inode, page, page->index);
 
+       snapc = detach_page_private(page);
        ceph_put_wrbuffer_cap_refs(ci, 1, snapc);
        ceph_put_snap_context(snapc);
-       page->private = 0;
-       ClearPagePrivate(page);
 }
 
 static int ceph_releasepage(struct page *page, gfp_t gfp)
@@ -589,8 +587,8 @@ static int writepage_nounlock(struct page *page, struct writeback_control *wbc)
                dout("writepage cleaned page %p\n", page);
                err = 0;  /* vfs expects us to return 0 */
        }
-       page->private = 0;
-       ClearPagePrivate(page);
+       oldest = detach_page_private(page);
+       WARN_ON_ONCE(oldest != snapc);
        end_page_writeback(page);
        ceph_put_wrbuffer_cap_refs(ci, 1, snapc);
        ceph_put_snap_context(snapc);  /* page's reference */
@@ -682,11 +680,9 @@ static void writepages_finish(struct ceph_osd_request *req)
                                clear_bdi_congested(inode_to_bdi(inode),
                                                    BLK_RW_ASYNC);
 
-                       ceph_put_snap_context(page_snap_context(page));
-                       page->private = 0;
-                       ClearPagePrivate(page);
-                       dout("unlocking %p\n", page);
+                       ceph_put_snap_context(detach_page_private(page));
                        end_page_writeback(page);
+                       dout("unlocking %p\n", page);
 
                        if (remove_page)
                                generic_error_remove_page(inode->i_mapping,