linux-kernel - Re: [PATCH/RFC] VFS: support parallel updates in the one directory.

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Date:   Thu, 24 Feb 2022 16:56:55 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     "Dave Chinner" <david@...morbit.com>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Linux NFS Mailing List" <linux-nfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Daire Byrne" <daire@...g.com>,
        "Andreas Dilger" <adilger.kernel@...ger.ca>
Subject: Re: [PATCH/RFC] VFS: support parallel updates in the one directory.

On Thu, 24 Feb 2022, Darrick J. Wong wrote:
> On Wed, Feb 23, 2022 at 09:45:46AM +1100, Dave Chinner wrote:
> > On Tue, Feb 22, 2022 at 01:24:50PM +1100, NeilBrown wrote:
> > > 
> > > Hi Al,
> > >  I wonder if you might find time to have a look at this patch.  It
> > >  allows concurrent updates to a single directory.  This can result in
> > >  substantial throughput improvements when the application uses multiple
> > >  threads to create lots of files in the one directory, and there is
> > >  noticeable per-create latency, as there can be with NFS to a remote
> > >  server.
> > > Thanks,
> > > NeilBrown
> > > 
> > > Some filesystems can support parallel modifications to a directory,
> > > either because the modification happen on a remote server which does its
> > > own locking (e.g.  NFS) or because they can internally lock just a part
> > > of a directory (e.g.  many local filesystems, with a bit of work - the
> > > lustre project has patches for ext4 to support concurrent updates).
> > > 
> > > To allow this, we introduce VFS support for parallel modification:
> > > unlink (including rmdir) and create.  Parallel rename is not (yet)
> > > supported.
> > 
> > Yay!
> > 
> > > If a filesystem supports parallel modification in a given directory, it
> > > sets S_PAR_UNLINK on the inode for that directory.  lookup_open() and
> > > the new lookup_hash_modify() (similar to __lookup_hash()) notice the
> > > flag and take a shared lock on the directory, and rely on a lock-bit in
> > > d_flags, much like parallel lookup relies on DCACHE_PAR_LOOKUP.
> > 
> > I suspect that you could enable this for XFS right now. XFS has internal
> > directory inode locking that should serialise all reads and writes
> > correctly regardless of what the VFS does. So while the VFS might
> > use concurrent updates (e.g. inode_lock_shared() instead of
> > inode_lock() on the dir inode), XFS has an internal metadata lock
> > that will then serialise the concurrent VFS directory modifications
> > correctly....
> 
> I don't think that will work because xfs_readdir doesn't hold the
> directory ILOCK while it runs, which means that readdir will see garbage
> if other threads now only hold inode_lock_shared while they update the
> directory.

I added this:
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -87,6 +87,7 @@ xfs_inode_alloc(
 	/* VFS doesn't initialise i_mode or i_state! */
 	VFS_I(ip)->i_mode = 0;
 	VFS_I(ip)->i_state = 0;
+	VFS_I(ip)->i_flags |= S_PAR_UPDATE;
 	mapping_set_large_folios(VFS_I(ip)->i_mapping);
 
 	XFS_STATS_INC(mp, vn_active);

and ran my highly sophisticated test in an XFS directory:

 for i in {1..70}; do ( for j in {1000..8000}; do touch $j; rm -f $j ; done ) & done

This doesn't crash - which is a good sign.
While that was going I tried
 while : ; do ls -l ; done

it sometimes reports garbage for the stat info:

total 0
-????????? ? ?    ?    ?            ? 1749
-????????? ? ?    ?    ?            ? 1764
-????????? ? ?    ?    ?            ? 1765
-rw-r--r-- 1 root root 0 Feb 24 16:47 1768
-rw-r--r-- 1 root root 0 Feb 24 16:47 1770
-rw-r--r-- 1 root root 0 Feb 24 16:47 1772
....

I *think* that is bad - probably the "garbage" that you referred to?

Obviously I gets lots of 
ls: cannot access '1764': No such file or directory
ls: cannot access '1749': No such file or directory
ls: cannot access '1780': No such file or directory
ls: cannot access '1765': No such file or directory

but that is normal and expected when you are creating and deleting
files during the ls.

NeilBrown




> 
> --D
> 
> > Yeah, I know, this isn't true concurrent dir updates, but it should
> > allow multiple implementations of the concurrent dir update VFS APIs
> > across multiple filesystems and shake out any assumptions that might
> > arise from a single implementation target (e.g. silly rename
> > quirks).
> > 
> > Cheers,
> > 
> > Dave.
> > -- 
> > Dave Chinner
> > david@...morbit.com
> 
>