[PATCH 2/6] VM/NFS: The VM must tell the filesystem when to free reclaimable pages

[Trond Myklebust <Trond.Myklebust-HgOvQuBEEgTQT0dZR+AlfA@public.gmane.org>]

balance_dirty_pages() should really tell the filesystem whether or not it
has an excess of actual dirty pages, or whether it would be more useful to
start freeing up the unstable writes.

Assume that if the number of unstable writes is more than 1/2 the number of
reclaimable pages, then we should force NFS to free up the former.

Signed-off-by: Trond Myklebust <Trond.Myklebust-HgOvQuBEEgTQT0dZR+AlfA@public.gmane.org>
Acked-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
---

 fs/nfs/write.c            |    2 +-
 include/linux/writeback.h |    5 +++++
 mm/page-writeback.c       |    9 ++++++++-
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 910be28..ee3daf4 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1417,7 +1417,7 @@ int nfs_commit_unstable_pages(struct address_space *mapping,
 	/* Don't commit yet if this is a non-blocking flush and there are
 	 * outstanding writes for this mapping.
 	 */
-	if (wbc->sync_mode != WB_SYNC_ALL &&
+	if (!wbc->force_commit && wbc->sync_mode != WB_SYNC_ALL &&
 	    radix_tree_tagged(&NFS_I(inode)->nfs_page_tree,
 		    NFS_PAGE_TAG_LOCKED)) {
 		mark_inode_unstable_pages(inode);
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 76e8903..3fd5c3e 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -62,6 +62,11 @@ struct writeback_control {
 	 * so we use a single control to update them
 	 */
 	unsigned no_nrwrite_index_update:1;
+	/*
+	 * The following is used by balance_dirty_pages() to
+	 * force NFS to commit unstable pages.
+	 */
+	unsigned force_commit:1;
 };
 
 /*
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0b19943..ede5356 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -485,6 +485,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 {
 	long nr_reclaimable, bdi_nr_reclaimable;
 	long nr_writeback, bdi_nr_writeback;
+	long nr_unstable_nfs;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
@@ -505,8 +506,9 @@ static void balance_dirty_pages(struct address_space *mapping,
 		get_dirty_limits(&background_thresh, &dirty_thresh,
 				&bdi_thresh, bdi);
 
+		nr_unstable_nfs = global_page_state(NR_UNSTABLE_NFS);
 		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
-					global_page_state(NR_UNSTABLE_NFS);
+					nr_unstable_nfs;
 		nr_writeback = global_page_state(NR_WRITEBACK);
 
 		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
@@ -537,6 +539,11 @@ static void balance_dirty_pages(struct address_space *mapping,
 		 * up.
 		 */
 		if (bdi_nr_reclaimable > bdi_thresh) {
+			wbc.force_commit = 0;
+			/* Force NFS to also free up unstable writes. */
+			if (nr_unstable_nfs > nr_reclaimable / 2)
+				wbc.force_commit = 1;
+
 			writeback_inodes_wbc(&wbc);
 			pages_written += write_chunk - wbc.nr_to_write;
 			get_dirty_limits(&background_thresh, &dirty_thresh,

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