[PATCH] fix failure to write ext2 indirects under load

This patch fixes a filesystem corrupting bug, present in 2.5.41 through
2.5.44.  It can cause ext2 indirect blocks to not be written out.  A
fsck will fix it up.

Under heavy memory pressure a PF_MEMALLOC task attemtps to write out a
blockdev page whose buffers are already under writeback and which were
dirtied while under writeback.

The writepage call returns -EAGAIN but because the caller is
PF_MEMALLOC, the page was not being marked dirty again.

The page sits on mapping->clean_pages for ever and it not written out.

The fix is to mark that page dirty again for all callers, regardless of
PF_MEMALLOC state.

fs/mpage.c

@@ -595,6 +595,10 @@ mpage_writepages(struct address_space *mapping,
 					test_clear_page_dirty(page)) {
 			if (writepage) {
 				ret = (*writepage)(page);
+				if (ret == -EAGAIN) {
+					__set_page_dirty_nobuffers(page);
+					ret = 0;
+				}
 			} else {
 				bio = mpage_writepage(bio, page, get_block,
 						&last_block_in_bio, &ret);
@@ -605,10 +609,6 @@ mpage_writepages(struct address_space *mapping,
 					pagevec_deactivate_inactive(&pvec);
 				page = NULL;
 			}
-			if (ret == -EAGAIN && page) {
-				__set_page_dirty_nobuffers(page);
-				ret = 0;
-			}
 			if (ret || (--(wbc->nr_to_write) <= 0))
 				done = 1;
 			if (wbc->nonblocking && bdi_write_congested(bdi)) {