linux-ext4 - Re: [PATCH 07/13] xfs: Convert to use invalidate

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Date:   Wed, 26 May 2021 15:42:32 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        ceph-devel@...r.kernel.org, Chao Yu <yuchao0@...wei.com>,
        Damien Le Moal <damien.lemoal@....com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Johannes Thumshirn <jth@...nel.org>,
        linux-cifs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>,
        Steve French <sfrench@...ba.org>, Ted Tso <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 07/13] xfs: Convert to use invalidate_lock

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@....de>
> > > CC: <linux-xfs@...r.kernel.org>
> > > CC: "Darrick J. Wong" <darrick.wong@...cle.com>
> > > Signed-off-by: Jan Kara <jack@...e.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@...e.com>
SUSE Labs, CR