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.

Fix this by introducing a new kiocb flag, IOCB_SWAP, that's set by the swap
code to indicate that the swapper is doing this operation and so overrule
the check in generic_write_checks().

Without this patch, the following is seen:

	Write error on dio swapfile (3800334336)

Altering __swap_writepage() to show the error shows:

	Write error (-26) on dio swapfile (3800334336)

Tested by swapping off all swap partitions and then swapping on a prepared
NFS file (CONFIG_NFS_SWAP=y is also needed).  Enough copies of the
following program then need to be run to force swapping to occur (at least
one per gigabyte of RAM):

	#include <stdbool.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <sys/mman.h>
	int main()
	{
		unsigned int pid = getpid(), iterations = 0;
		size_t i, j, size = 1024 * 1024 * 1024;
		char *p;
		bool mismatch;
		p = malloc(size);
		if (!p) {
			perror("malloc");
			exit(1);
		}
		srand(pid);
		for (i = 0; i < size; i += 4)
			*(unsigned int *)(p + i) = rand();
		do {
			for (j = 0; j < 16; j++) {
				for (i = 0; i < size; i += 4096)
					*(unsigned int *)(p + i) += 1;
				iterations++;
			}
			mismatch = false;
			srand(pid);
			for (i = 0; i < size; i += 4) {
				unsigned int r = rand();
				unsigned int v = *(unsigned int *)(p + i);
				if (i % 4096 == 0)
					v -= iterations;
				if (v != r) {
					fprintf(stderr, "mismatch %zx: %x != %x (diff %x)\n",
						i, v, r, v - r);
					mismatch = true;
				}
			}
		} while (!mismatch);
		exit(1);
	}


Fixes: dc617f29dbe5 ("vfs: don't allow writes to swap files")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Darrick J. Wong <redacted>
cc: Christoph Hellwig <hch@lst.de>
cc: Trond Myklebust <redacted>
cc: linux-nfs@vger.kernel.org
---

 fs/read_write.c    |    2 +-
 include/linux/fs.h |    1 +
 mm/page_io.c       |    7 ++++---
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 9db7adf160d2..daef721ca67e 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c

@@ -1646,7 +1646,7 @@ ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 	loff_t count;
 	int ret;
 
-	if (IS_SWAPFILE(inode))
+	if (IS_SWAPFILE(inode) && !(iocb->ki_flags & IOCB_SWAP))
 		return -ETXTBSY;
 
 	if (!iov_iter_count(from))

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 640574294216..b3e6a20f28ef 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -319,6 +319,7 @@ enum rw_hint {
 /* iocb->ki_waitq is valid */
 #define IOCB_WAITQ		(1 << 19)
 #define IOCB_NOIO		(1 << 20)
+#define IOCB_SWAP		(1 << 21)	/* This is a swap request */
 
 struct kiocb {
 	struct file		*ki_filp;

diff --git a/mm/page_io.c b/mm/page_io.c
index d597bc6e6e45..edb72bf624d2 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c

@@ -303,7 +303,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 
 		iov_iter_bvec(&from, WRITE, &bv, 1, PAGE_SIZE);
 		init_sync_kiocb(&kiocb, swap_file);
-		kiocb.ki_pos = page_file_offset(page);
+		kiocb.ki_pos	= page_file_offset(page);
+		kiocb.ki_flags	= IOCB_DIRECT | IOCB_WRITE | IOCB_SWAP;
 
 		set_page_writeback(page);
 		unlock_page(page);

@@ -324,8 +325,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 			 */
 			set_page_dirty(page);
 			ClearPageReclaim(page);
-			pr_err_ratelimited("Write error on dio swapfile (%llu)\n",
-					   page_file_offset(page));
+			pr_err_ratelimited("Write error (%d) on dio swapfile (%llu)\n",
+					   ret, page_file_offset(page));
 		}
 		end_page_writeback(page);
 		return ret;