linux-kernel - Re: [PATCH v2] vfs: Don't exchange "short" filenames unconditionally.

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Message-ID: <20140927194558.GV7996@ZenIV.linux.org.uk>
Date:	Sat, 27 Sep 2014 20:45:58 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mikhail Efremov <sem@...linux.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] vfs: Don't exchange "short" filenames unconditionally.

On Sat, Sep 27, 2014 at 08:16:57PM +0100, Al Viro wrote:
> On Sat, Sep 27, 2014 at 07:31:39PM +0100, Al Viro wrote:
> 
> > We can get the long name cases right, and I agree that it'll make the
> > things nicer, but it might take a couple of days to get right.  The thing
> > I'm concerned about is not screwing DCACHE_RCUACCESS up.
> 
> FWIW, I suspect that the right approach is to put refcount + rcu_head in
> front of external name and do the following:
> 	* __d_free() checks if we have an external name, gets its containing
> structure and does if (atomic_dec_and_test(&name->count)) kfree(name);
> 	* switch_names() in non-exchange case (I'd probably call it copy_name,
> not move_names, but anyway) sets DCACHE_RCUACCESS on target (source has
> already gotten it from __d_rehash()), increments refcount on target's name
> if external and, if the source old name is external, decrements its refcount
> and calls kfree_rcu() if it has hit zero.
> 
> AFAICS, it guarantees that we'll schedule an RCU callback on name's rch_head
> at most once, that we won't free it while RCU callback on it is scheduled
> and we won't free it until a grace period has expired since the last time
> it had been referenced by observable dentries.  Do you see any holes in that?

We probably want to put a union of refcount and rcu_head there, actually...
Gives the right alignment without padding.  As in
struct ext_name {
	union {
		atomic_t count;
		struct rcu_head head;
	};
	char name[0];
};
->count corresponds to the number of dentries that have ->d_name.name
pointing to the sucker's ->name.  And we use ->head only when it reaches
zero in __d_move().  That's 2 words per external name; somewhat unpleasant
on 64bit, but I don't see how to avoid an rcu_head in there...  The cutoff
for external names is 32 bytes on 64bit boxen.  That way we get 16 bytes
of overhead per long-named dentry...  OTOH, we allocate them with kmalloc(),
so it means that 32-character names lead to 64-bytes actual allocation.

Hmmm...  So the old behaviour is
 32--63 	=> 64 byte allocation
 64--95 	=> 96
 96--127	=> 128
and the new one
 32--47 	=> 64 byte allocation
 48--79 	=> 96
 80--111	=> 128
112--127	=> 192
(components longer than 128 characters are definitely too rare to worry about)
IOW, the main worry is about the names in range from 48 to 64 characters;
for those we push the allocation from size-64 to size-96...

Note, BTW, that git hits external name case on everything except 32-bit UP;
a _lot_ of 38-character names there.  And IIRC there had been some plans for
possible replacement of SHA1 with something wider, right?
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