Re: [PATCH 07/13] xfs: Convert to use invalidate_lock

Dave Chinner
2021-05-25T21:40:52Z

On Tue, May 25, 2021 at 03:50:44PM +0200, Jan Kara wrote:
> Use invalidate_lock instead of XFS internal i_mmap_lock. The intended
> purpose of invalidate_lock is exactly the same. Note that the locking in
> __xfs_filemap_fault() slightly changes as filemap_fault() already takes
> invalidate_lock.
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> CC: <linux-xfs@vger.kernel.org>
> CC: "Darrick J. Wong" <darrick.wong@oracle.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/xfs/xfs_file.c | 12 ++++++-----
> fs/xfs/xfs_inode.c | 52 ++++++++++++++++++++++++++--------------------
> fs/xfs/xfs_inode.h | 1 -
> fs/xfs/xfs_super.c | 2 --
> 4 files changed, 36 insertions(+), 31 deletions(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 396ef36dcd0a..dc9cb5c20549 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1282,7 +1282,7 @@ xfs_file_llseek(
> *
> * mmap_lock (MM)
> * sb_start_pagefault(vfs, freeze)
> - * i_mmaplock (XFS - truncate serialisation)
> + * invalidate_lock (vfs/XFS_MMAPLOCK - truncate serialisation)
> * page_lock (MM)
> * i_lock (XFS - extent map serialisation)
> */
> @@ -1303,24 +1303,26 @@ __xfs_filemap_fault(
> file_update_time(vmf->vma->vm_file);
> }
>
> - xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> if (IS_DAX(inode)) {
> pfn_t pfn;
>
> + xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> ret = dax_iomap_fault(vmf, pe_size, &pfn, NULL,
> (write_fault && !vmf->cow_page) ?
> &xfs_direct_write_iomap_ops :
> &xfs_read_iomap_ops);
> if (ret & VM_FAULT_NEEDDSYNC)
> ret = dax_finish_sync_fault(vmf, pe_size, pfn);
> + xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> } else {
> - if (write_fault)
> + if (write_fault) {
> + xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> ret = iomap_page_mkwrite(vmf,
> &xfs_buffered_write_iomap_ops);
> - else
> + xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> + } else
> ret = filemap_fault(vmf);
> }
> - xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);

This seems kinda messy. filemap_fault() basically takes the
invalidate lock around the entire operation, it runs, so maybe it
would be cleaner to implement it as:

filemap_fault_locked(vmf)
{
/* does the filemap fault work */
}

filemap_fault(vmf)
{
filemap_invalidate_down_read(...)
ret = filemap_fault_locked(vmf)
filemap_invalidate_up_read(...)
return ret;
}

And that means XFS could just call filemap_fault_locked() and not
have to do all this messy locking just to avoid holding the lock
that filemap_fault has now internalised.

> @@ -355,8 +358,11 @@ xfs_isilocked(
>
> if (lock_flags & (XFS_MMAPLOCK_EXCL|XFS_MMAPLOCK_SHARED)) {
> if (!(lock_flags & XFS_MMAPLOCK_SHARED))
> - return !!ip->i_mmaplock.mr_writer;
> - return rwsem_is_locked(&ip->i_mmaplock.mr_lock);
> + return !debug_locks ||
> + lockdep_is_held_type(
> + &VFS_I(ip)->i_mapping->invalidate_lock,
> + 0);
> + return rwsem_is_locked(&VFS_I(ip)->i_mapping->invalidate_lock);
> }

<sigh>

And so here we are again, losing more of our read vs write debug
checks on debug kernels when lockdep is not enabled....

Can we please add rwsem_is_locked_read() and rwsem_is_locked_write()
wrappers that just look at the rwsem counter value to determine how
the lock is held? Then the mrlock_t can go away entirely....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com