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

David Howells
2021-08-12T20:22:25Z

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 <darrick.wong@oracle.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Trond Myklebust <trond.myklebust@primarydata.com>
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;