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

Jan Kara
2021-05-26T10:21:07Z

On Wed 26-05-21 07:40:41, Dave Chinner wrote:
> 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.

Sure, I can do that.

> > @@ -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....

Apparently someone already did that for XFS as Darrick pointed out. So we
just have to sort out how to merge it.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR