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    SubjectRe: [opensuse] nfs_update_inode: inode X mode changed, Y to Z
    On Wed, March 26, 2008 9:13 am, Josef 'Jeff' Sipek wrote:
    > 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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