Re: [RFC PATCH v2 1/5] nfs: Fix write to swapfile failure due to generic_write_checks()

[Matthew Wilcox <willy@infradead.org>]

On Thu, Aug 12, 2021 at 09:22:06PM +0100, David Howells wrote:
> Trying to use a swapfile on NFS results in every DIO write failing with
> ETXTBSY because generic_write_checks(), as called by nfs_direct_write()
> from nfs_direct_IO(), forbids writes to swapfiles.

Why does nfs_direct_write() call generic_write_checks()?

ie call generic_write_checks() earlier, and only swap would bypass them.

diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index 2e894fec036b..7e2ca6b5fc5f 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -905,9 +905,6 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter)
 	dfprintk(FILE, "NFS: direct write(%pD2, %zd@%Ld)\n",
 		file, iov_iter_count(iter), (long long) iocb->ki_pos);
 
-	result = generic_write_checks(iocb, iter);
-	if (result <= 0)
-		return result;
 	count = result;
 	nfs_add_stats(mapping->host, NFSIOS_DIRECTWRITTENBYTES, count);
 
diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 1fef107961bc..91b2e3214836 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -611,6 +611,10 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 	errseq_t since;
 	int error;
 
+	result = generic_write_checks(iocb, from);
+	if (result < 0)
+		return result;
+
 	result = nfs_key_timeout_notify(file, inode);
 	if (result)
 		return result;
@@ -621,8 +625,6 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 	dprintk("NFS: write(%pD2, %zu@%Ld)\n",
 		file, iov_iter_count(from), (long long) iocb->ki_pos);
 
-	if (IS_SWAPFILE(inode))
-		goto out_swapfile;
 	/*
 	 * O_APPEND implies that we must revalidate the file length.
 	 */
@@ -636,7 +638,6 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 
 	since = filemap_sample_wb_err(file->f_mapping);
 	nfs_start_io_write(inode);
-	result = generic_write_checks(iocb, from);
 	if (result > 0) {
 		current->backing_dev_info = inode_to_bdi(inode);
 		result = generic_perform_write(file, from, iocb->ki_pos);
@@ -677,10 +678,6 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 	nfs_add_stats(inode, NFSIOS_NORMALWRITTENBYTES, written);
 out:
 	return result;
-
-out_swapfile:
-	printk(KERN_INFO "NFS: attempt to write to active swap file!\n");
-	return -ETXTBSY;
 }
 EXPORT_SYMBOL_GPL(nfs_file_write);