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

From: Al Viro
Date: Sat Sep 27 2014 - 15:46:07 EST

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 ->
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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