Re: [PATCH 10/18] NFS: swap IO handling is slightly different for O_DIRECT IO
From: Mark Hemment <hidden>
Date: 2021-12-20 15:23:18
Also in:
linux-mm, lkml
On Thu, 16 Dec 2021 at 23:58, NeilBrown [off-list ref] 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 <redacted> ---
...
quoted hunk ↗ jump to hunk
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