On Wed, March 26, 2008 9:13 am, Josef 'Jeff' Sipek wrote:
quoted text
> On Wed, Mar 26, 2008 at 08:38:22AM +1100, NeilBrown wrote:
> ...
>> However you still need to do something about the generation number.  It
>> must be set to something.
>
> Right.
>
>> When you allocate an inode that doesn't currently exist on the device,
>> you obviously cannot increment the old value and use that.
>
> Makes sense.
>
>> However you can do a lot better than always using 0.
>
> I looked at the code (xfs_ialloc.c:xfs_ialloc_ag_alloc)
>
>  290                 /*
>  291                  * Set initial values for the inodes in this buffer.
>  292                  */
>  293                 xfs_biozero(fbuf, 0, ninodes <<
> args.mp->m_sb.sb_inodelog);
>  294                 for (i = 0; i < ninodes; i++) {
>  295                         free = XFS_MAKE_IPTR(args.mp, fbuf, i);
>  296                         free->di_core.di_magic =
> cpu_to_be16(XFS_DINODE_MAGIC);
>  297                         free->di_core.di_version = version;
>  298                         free->di_next_unlinked =
> cpu_to_be32(NULLAGINO);
>  299                         xfs_ialloc_log_di(tp, fbuf, i,
>  300                                 XFS_DI_CORE_BITS |
> XFS_DI_NEXT_UNLINKED);
>  301                 }
>
> xfs_biozero(...) turns into a memset(buf, 0, len), and since the loop that
> follows doesn't change the generation number, it'll stay 0.
>
>> The simplest would be to generate a 'random' number (get_random_bytes).
>> Slightly better would be to generate a random number at boot time
>> and use that, incrementing it each time it is used to set the
>> generation number for an inode.
>
> I'm not familiar enough with NFS, do you want something that's
> monotonically
> increasing or do you just test for inequality?  If it is inequality, why
> not
> just use something like the jiffies - that should be unique enough.
>

What we need is for the "filehandle" to be stable and unique.
By 'stable' I mean that every time I get the filehandle for a particular
file, I get the same string of bytes.
By 'uniqie' I mean that if I get two filehandles for two different
files, they must differ in at least one bit.
If a file is deleted and the inode is re-used for a new file, then the
old and new files are different and must have different file handles.

The filehandle is traditionally generated from the inode number and
a generation number, but the filesystem can actually do whatever it
likes.  xfs does it with xfs_fs_encode_fh().

Certainly you could initialise the i_generation to jiffies in
xfs_ialloc_ag_alloc.  That would be a suitable fix.  get_random_bytes
might be better, but the difference probably wouldn't be noticeable.

NeilBrown

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