Subject: Re: 9p fs-cache tests/benchmark (was: 9p fscache Duplicate cookie detected)

From: [email-masked]
Date: Sat, 2 Apr 2022 08:11:04 +0900

Christian Schoenebeck wrote on Fri, Apr 01, 2022 at 04:19:20PM +0200:
> 4. v9fs_qid_iget_dotl [fs/9p/vfs_inode_dotl.c, 133]:
> 
> 	v9fs_cache_inode_get_cookie(inode);
> 	^--- Called independent of function argument "new"'s value here
>    https://github.com/torvalds/linux/blob/e8b767f5e04097aaedcd6e06e2270f9fe5282696/fs/9p/vfs_inode_dotl.c#L133

uh, I'd have assumed either this call or the function to check
v9ses->cache, but it doesn't look like either do...
There's a nice compile-time static inline empty definition if FSCACHE is
not compiled in, but that should -also- be a check at runtime based on
the session struct.

For your remark vs. the 'new' argument, it does depend on it:
 - new determines which check is used for iget5_locked.
In the 'new' case, v9fs_test_new_inode_dotl always returns 0 so we
always get a new inode.
 - if iget5_locked found an existing inode (i_state & I_NEW false) we
return it.
 - at this point we're allocating a new inode, so we should initialize
its cookie if it's on a fscache-enabled mount

So that part looks OK to me.

What isn't correct with qemu setting qid version is the non-new case's
v9fs_test_inode_dotl, it'll consider the inode to be new if version
changed so it would recreate new, different inodes with same inode
number/cookie and I was sure that was the problem, but it looks like
there's more to it from your reply below :(

>> Well, at least that one is an easy fix: we just don't need this.
>> It's the easiest way to reproduce but there are also harder to fix
>> collisions, file systems only guarantee unicity for (fsid,inode
>> number,version) which is usually bigger than 128 bits (although version
>> is often 0), but version isn't exposed to userspace easily...
>> What we'd want for unicity is handle from e.g. name_to_handle_at but
>> that'd add overhead, wouldn't fit in qid path and not all fs are capable
>> of providing one... The 9p protocol just doesn't want bigger handles
>> than qid path.
> 
> No bigger qid.path on 9p protocol level in future? Why?

I have no idea about the "9p protocol" as a standard, nor who decides
how that evolves -- I guess if we wanted to we could probably make a
9p2022.L without concerting much around, but I have no plan to do
that... But if we do, I can probably add quite a few things to the
list of things that might need to change :)

That being said, this particular change of qid format is rather
annoying. 9p2000.L basically just added new message types, so dissectors
such as wireshark could barge in the middle of the tcp flow and more or
less understand; modifying a basic type like this would require to
either catch the TVERSION message or make new message types for
everything that deals wth qids (auth/attach, walk, lopen, lcreate,
mknod, getattr, readdir, mkdir... that's quite a few)

So I think we're better off with the status quo here.

>> And, err, looking at the qemu code
>> 
>>   qidp->version = stbuf->st_mtime ^ (stbuf->st_size << 8);
>> 
>> so the qid is treated as "data version",
>> but on kernel side we've treated it as inode version (i_version, see
>> include/linux/iversion.h)
>> 
>> (v9fs_test_inode_dotl checks the version is the same when comparing two
>> inodes) so it will incorrectly identify two identical inodes as
>> different.
>> That will cause problems...
>> Since you'll be faster than me could you try keeping it at 0 there?
> 
> I tried with your suggested change on QEMU side:
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index a6d6b3f835..5d9be87758 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -981,7 +981,7 @@ static int stat_to_qid(V9fsPDU *pdu, const struct stat *stbuf, V9fsQID *qidp)
>          memcpy(&qidp->path, &stbuf->st_ino, size);
>      }
>  
> -    qidp->version = stbuf->st_mtime ^ (stbuf->st_size << 8);
> +    qidp->version = 0;
>      qidp->type = 0;
>      if (S_ISDIR(stbuf->st_mode)) {
>          qidp->type |= P9_QID_TYPE_DIR;
> 
> Unfortunately it did not make any difference for these 2 Linux kernel fs-cache
> issues at least; still same errors, and same suboptimal performance.

Thanks, I'll give it a try today or tomorrow, adding some trace when we
create inodes might give a clue...

-- 
Dominique