On Thu, 16 Dec 2021 at 23:58, NeilBrown <neilb@xxxxxxx> wrote:
>
> 1/ Taking the i_rwsem for swap IO triggers lockdep warnings regarding
> possible deadlocks with "fs_reclaim". These deadlocks could, I believe,
> eventuate if a buffered read on the swapfile was attempted.
>
> We don't need coherence with the page cache for a swap file, and
> buffered writes are forbidden anyway. There is no other need for
> i_rwsem during direct IO. So never take it for swap_rw()
Agreed. I cannot see an issue with not taking the sem.
> 2/ generic_write_checks() explicitly forbids writes to swap, and
> performs checks that are not needed for swap. So bypass it
> for swap_rw().
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
...
> diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> @@ -625,7 +625,7 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
> return result;
>
> if (iocb->ki_flags & IOCB_DIRECT)
> - return nfs_file_direct_write(iocb, from);
> + return nfs_file_direct_write(iocb, from, false);
>
> dprintk("NFS: write(%pD2, %zu@%Ld)\n",
> file, iov_iter_count(from), (long long) iocb->ki_pos);
This code at the top of nfs/file.c should be removed;
/* Hack for future NFS swap support */
#ifndef IS_SWAPFILE
# define IS_SWAPFILE(inode) (0)
#endif
As IS_SWAPFILE() is used just below this diff (to prevent buffered
writes), better be using a non-hacked version.
Mark
