On Fri, 2017-08-18 at 17:12 +1000, NeilBrown wrote:
> Any dirty NFS page holds an s_active reference on the superblock,
> because page_private() references an nfs_page, which references an
> open context, which references the superblock.
>
> So if there are any dirty pages when the filesystem is unmounted, the
> unmount will act like a "lazy" unmount and not call ->kill_sb().
> Background write-back can then write out the pages *after* the
> filesystem
> unmount has apparently completed.
>
> This contrasts with other filesystems which do not hold extra
> s_active
> references, so ->kill_sb() is reliably called on unmount, and
> generic_shutdown_super() will call sync_filesystem() to flush
> everything out before the unmount completes.
>
> When open/write/close is used to modify files, the final close causes
> f_op->flush to be called, which flushes all dirty pages. However if
> open/mmap/close/modify-memory/unmap is used, dirty pages can remain
> in
> memory after the application has dropped all references to the file.
> Similarly if a loop-mount is done with a NFS file, there is no final
> flush when the loop device is destroyed and the file can have dirty
> data after the last "close".
>
> Also, a loop-back mount of a device does not "close" the file when
> the
> loop device is destroyed. This can leave dirty page cache pages too.
>
> Fix this by calling vfs_fsync() in nfs_file_release (aka
> f_op->release()). This means that on the final unmap of a file (or
> destruction of a loop device), all changes are flushed, and ensures
> that
> when unmount is requested there will be no dirty pages to delay the
> final unmount.
>
> Without this patch, it is not safe to stop or disconnect the NFS
> server after all clients have unmounted. They need to unmount and
> call "sync".
>
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> ---
> fs/nfs/file.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> index af330c31f627..aa883d8b24e6 100644
> --- a/fs/nfs/file.c
> +++ b/fs/nfs/file.c
> @@ -81,6 +81,12 @@ nfs_file_release(struct inode *inode, struct file
> *filp)
> {
> dprintk("NFS: release(%pD2)\n", filp);
>
> + if (filp->f_mode & FMODE_WRITE)
> + /* Ensure dirty mmapped pages are flushed
> + * so there will be no dirty pages to
> + * prevent an unmount from completing.
> + */
> + vfs_fsync(filp, 0);
> nfs_inc_stats(inode, NFSIOS_VFSRELEASE);
> nfs_file_clear_open_context(filp);
> return 0;
The right fix here is to ensure that umount() flushes the dirty data,
and that it aborts the umount attempt if the flush fails. Otherwise, a
signal to the above fsync call will cause the problem to reoccur.
--
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@xxxxxxxxxxxxxxx
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