[RFC PATCH 3/3] NFS: Enable the RWF_DONTCACHE flag for the NFS client

[trondmy@kernel.org]

From: Trond Myklebust <trond.myklebust@hammerspace.com>

While the NFS readahead code has no problems using the generic code to
manage the dropbehind behaviour enabled by RWF_DONTCACHE, the write code
needs to deal with the fact that NFS writeback uses a 2 step process
(UNSTABLE write followed by COMMIT).
This commit replaces the use of the folio dropbehind flag with a local
NFS request flag that triggers the dropbehind behaviour once the data
has been written to stable storage.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 fs/nfs/file.c            |  2 ++
 fs/nfs/nfs4file.c        |  2 ++
 fs/nfs/write.c           | 12 +++++++++++-
 include/linux/nfs_page.h |  1 +
 4 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 033feeab8c34..60d47f141acb 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -910,5 +910,7 @@ const struct file_operations nfs_file_operations = {
 	.splice_write	= iter_file_splice_write,
 	.check_flags	= nfs_check_flags,
 	.setlease	= simple_nosetlease,
+
+	.fop_flags	= FOP_DONTCACHE,
 };
 EXPORT_SYMBOL_GPL(nfs_file_operations);
diff --git a/fs/nfs/nfs4file.c b/fs/nfs/nfs4file.c
index 1cd9652f3c28..e6726499c585 100644
--- a/fs/nfs/nfs4file.c
+++ b/fs/nfs/nfs4file.c
@@ -467,4 +467,6 @@ const struct file_operations nfs4_file_operations = {
 #else
 	.llseek		= nfs_file_llseek,
 #endif
+
+	.fop_flags	= FOP_DONTCACHE,
 };
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 23df8b214474..e0ac439ab211 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -359,8 +359,12 @@ static void nfs_folio_end_writeback(struct folio *folio)
 static void nfs_page_end_writeback(struct nfs_page *req)
 {
 	if (nfs_page_group_sync_on_bit(req, PG_WB_END)) {
+		struct folio *folio = nfs_page_to_folio(req);
+
+		if (folio_test_clear_dropbehind(folio))
+			set_bit(PG_DROPBEHIND, &req->wb_flags);
 		nfs_unlock_request(req);
-		nfs_folio_end_writeback(nfs_page_to_folio(req));
+		nfs_folio_end_writeback(folio);
 	} else
 		nfs_unlock_request(req);
 }
@@ -813,6 +817,9 @@ static void nfs_inode_remove_request(struct nfs_page *req)
 			clear_bit(PG_MAPPED, &req->wb_head->wb_flags);
 		}
 		spin_unlock(&mapping->i_private_lock);
+
+		if (test_bit(PG_DROPBEHIND, &req->wb_flags))
+			folio_end_dropbehind(folio);
 	}
 
 	if (test_and_clear_bit(PG_INODE_REF, &req->wb_flags)) {
@@ -2093,6 +2100,7 @@ int nfs_wb_folio(struct inode *inode, struct folio *folio)
 		.range_start = range_start,
 		.range_end = range_start + len - 1,
 	};
+	bool dropbehind = folio_test_clear_dropbehind(folio);
 	int ret;
 
 	trace_nfs_writeback_folio(inode, range_start, len);
@@ -2113,6 +2121,8 @@ int nfs_wb_folio(struct inode *inode, struct folio *folio)
 			goto out_error;
 	}
 out_error:
+	if (dropbehind)
+		folio_set_dropbehind(folio);
 	trace_nfs_writeback_folio_done(inode, range_start, len, ret);
 	return ret;
 }
diff --git a/include/linux/nfs_page.h b/include/linux/nfs_page.h
index 169b4ae30ff4..1a017b5b476f 100644
--- a/include/linux/nfs_page.h
+++ b/include/linux/nfs_page.h
@@ -37,6 +37,7 @@ enum {
 	PG_REMOVE,		/* page group sync bit in write path */
 	PG_CONTENDED1,		/* Is someone waiting for a lock? */
 	PG_CONTENDED2,		/* Is someone waiting for a lock? */
+	PG_DROPBEHIND,		/* Implement RWF_DONTCACHE */
 };
 
 struct nfs_inode;
-- 
2.49.0