[Btrfs-devel] Some queries about map_extent_buffer()

[Peter Teoh]

On Mon, Mar 17, 2008 at 7:52 PM, Yan Zheng <yanzheng at 21cn.com> wrote:
> 2008/3/17, Peter Teoh <htmldeveloper at gmail.com>:
>
> > Thanks for the explanation.   But my point is the C statement "continue":
>  >
>  >         for (i = start_slot; i < end_slot; i++) {============> start
>  >  of loop, "continue" therefore will come here.
>  >                 int close = 1;
>  >
>  >
>  >                 if (!parent->map_token) {
>  >                         map_extent_buffer(parent,
>  >                                         btrfs_node_key_ptr_offset(i),
>  >                                         sizeof(struct btrfs_key_ptr),
>  >                                         &parent->map_token, &parent->kaddr,
>  >                                         &parent->map_start, &parent->map_len,
>  >                                         KM_USER1);
>  >                 }
>  Please pay attention to the fourth parameter for map_extent_buffer.
>  Once map_extent_buffer has been called,  parent->map_token must not be
>  NULL.

Yes, but I think u are assuming that map_extent_buffer() execute
correctly.   What if it is not?

Trace through map_extent_buffer()-->inside the function:

map_extent_buffer:
{
        int err;
        int save = 0;
        if (eb->map_token) {
                unmap_extent_buffer(eb, eb->map_token, km);
                eb->map_token = NULL;
                save = 1;
        }
        err = map_private_extent_buffer(eb, start, min_len, token, map,
                                       map_start, map_len, km);
        if (!err && save) {
                eb->map_token = *token;
                eb->kaddr = *map;
                eb->map_start = *map_start;
                eb->map_len = *map_len;
        }
        return err;
}

err can be EINVAL, because of this:

        if (i != end_i)
                return -EINVAL;

And this is returned even before assigning the token:

        *token = kaddr;

So if parent->map_token started off with NULL value, it will continue
to remain as NULL, thus leading to map_extent_buffer() being called
multiple times.   Correct?

>  The test 'if (!parent->map_token)' prevents map_extent_buffer from
>  being called again.
>
>  Regards
>  YZ
>



-- 
Regards,
Peter Teoh