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    SubjectRe: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland
    In message <1194280730.6271.145.camel@localhost>, Dave Hansen writes:
    > On Mon, 2007-11-05 at 15:40 +0000, Hugh Dickins wrote:
    [...]
    > I have a decent guess what the bug is, too. In the unionfs code:
    >
    > > int init_lower_nd(struct nameidata *nd, unsigned int flags)
    > > {
    > > ...
    > > #ifdef ALLOC_LOWER_ND_FILE
    > > file = kzalloc(sizeof(struct file), GFP_KERNEL);
    > > if (unlikely(!file)) {
    > > err = -ENOMEM;
    > > break; /* exit switch statement and thus return */
    > > }
    > > nd->intent.open.file = file;
    > > #endif /* ALLOC_LOWER_ND_FILE */
    >
    > The r/o bind mount code will mnt_drop_write() on that file's f_vfsmnt at
    > __fput() time. Since that code never got a write on the mount, we'll
    > see an imbalance if the file was opened for a write. I don't see this
    > file's mnt set anywhere, so I'm not completely sure that this is it. In
    > any case, rolling your own 'struct file' without using alloc_file() and
    > friends is a no-no.
    [...]

    This #ifdef'd code in unionfs is actually not enabled. I left it there as a
    reminder of possible future things to come (esp. if nameidata gets split).
    There's a related comment earlier in fs/unionfs/lookup.c:init_lower_nd()
    that says:

    #ifdef ALLOC_LOWER_ND_FILE
    /*
    * XXX: one day we may need to have the lower return an open file
    * for us. It is not needed in 2.6.23-rc1 for nfs2/nfs3, but may
    * very well be needed for nfs4.
    */
    struct file *file;
    #endif /* ALLOC_LOWER_ND_FILE */

    In the interest of keeping unionfs as simple as I can, when I implemented
    the whole "pass a lower nd" stuff, I left thos bits of semi-experimental
    #ifdef code for this lower file upon open-intent. It's not enabled and up
    until now, it didn't seem to be needed.

    Do you think unionfs has to start using this nd->intent.open.file stuff?

    Thanks,
    Erez.
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