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

Jan Kara
2021-05-26T13:42:39Z

On Wed 26-05-21 12:20:59, Jan Kara wrote:
> 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.

Hum, looking into this in more detail it isn't as easy. There are some
operations inside filemap_fault() that need to be done outside of
invalidate_lock. In particular we call into readahead code which will grab
invalidate_lock for itself. So we'd need to pass in struct
readahead_control whether invalidate_lock is held or not which is IMHO
uglier than what we currently do in __xfs_filemap_fault().

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