linux-kernel - Re: [PATCH RFC 6/7] libfs: Convert simple directory offsets to use a Maple Tree

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Message-ID: <20240215140244.njd5emd6ikbjfj27@quack3>
Date: Thu, 15 Feb 2024 15:02:44 +0100
From: Jan Kara <jack@...e.cz>
To: Chuck Lever <chuck.lever@...cle.com>
Cc: Jan Kara <jack@...e.cz>, Chuck Lever <cel@...nel.org>,
	viro@...iv.linux.org.uk, brauner@...nel.org, hughd@...gle.com,
	akpm@...ux-foundation.org, Liam.Howlett@...cle.com,
	oliver.sang@...el.com, feng.tang@...el.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	maple-tree@...ts.infradead.org, linux-mm@...ck.org, lkp@...el.com
Subject: Re: [PATCH RFC 6/7] libfs: Convert simple directory offsets to use a
 Maple Tree

On Thu 15-02-24 08:45:33, Chuck Lever wrote:
> On Thu, Feb 15, 2024 at 02:06:01PM +0100, Jan Kara wrote:
> > On Tue 13-02-24 16:38:01, Chuck Lever wrote:
> > > From: Chuck Lever <chuck.lever@...cle.com>
> > > 
> > > Test robot reports:
> > > > kernel test robot noticed a -19.0% regression of aim9.disk_src.ops_per_sec on:
> > > >
> > > > commit: a2e459555c5f9da3e619b7e47a63f98574dc75f1 ("shmem: stable directory offsets")
> > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > > 
> > > Feng Tang further clarifies that:
> > > > ... the new simple_offset_add()
> > > > called by shmem_mknod() brings extra cost related with slab,
> > > > specifically the 'radix_tree_node', which cause the regression.
> > > 
> > > Willy's analysis is that, over time, the test workload causes
> > > xa_alloc_cyclic() to fragment the underlying SLAB cache.
> > > 
> > > This patch replaces the offset_ctx's xarray with a Maple Tree in the
> > > hope that Maple Tree's dense node mode will handle this scenario
> > > more scalably.
> > > 
> > > In addition, we can widen the directory offset to an unsigned long
> > > everywhere.
> > > 
> > > Suggested-by: Matthew Wilcox <willy@...radead.org>
> > > Reported-by: kernel test robot <oliver.sang@...el.com>
> > > Closes: https://lore.kernel.org/oe-lkp/202309081306.3ecb3734-oliver.sang@intel.com
> > > Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
> > 
> > OK, but this will need the performance numbers.
> 
> Yes, I totally concur. The point of this posting was to get some
> early review and start the ball rolling.
> 
> Actually we expect roughly the same performance numbers now. "Dense
> node" support in Maple Tree is supposed to be the real win, but
> I'm not sure it's ready yet.
> 
> 
> > Otherwise we have no idea
> > whether this is worth it or not. Maybe you can ask Oliver Sang? Usually
> > 0-day guys are quite helpful.
> 
> Oliver and Feng were copied on this series.
> 
> 
> > > @@ -330,9 +329,9 @@ int simple_offset_empty(struct dentry *dentry)
> > >  	if (!inode || !S_ISDIR(inode->i_mode))
> > >  		return ret;
> > >  
> > > -	index = 2;
> > > +	index = DIR_OFFSET_MIN;
> > 
> > This bit should go into the simple_offset_empty() patch...
> > 
> > > @@ -434,15 +433,15 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > >  
> > >  	/* In this case, ->private_data is protected by f_pos_lock */
> > >  	file->private_data = NULL;
> > > -	return vfs_setpos(file, offset, U32_MAX);
> > > +	return vfs_setpos(file, offset, MAX_LFS_FILESIZE);
> > 					^^^
> > Why this? It is ULONG_MAX << PAGE_SHIFT on 32-bit so that doesn't seem
> > quite right? Why not use ULONG_MAX here directly?
> 
> I initially changed U32_MAX to ULONG_MAX, but for some reason, the
> length checking in vfs_setpos() fails. There is probably a sign
> extension thing happening here that I don't understand.

Right. loff_t is signed (long long). So I think you should make the
'offset' be long instead of unsigned long and allow values 0..LONG_MAX?
Then you can pass LONG_MAX here. You potentially loose half of the usable
offsets on 32-bit userspace with 64-bit file offsets but who cares I guess?

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