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@xxxxxxxxxxxxx
>
>
