[Btrfs-devel] Some queries about map_extent_buffer()

[Yan Zheng]

2008/3/17, Peter Teoh <htmldeveloper at gmail.com>:
> 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?
>
map_extent_buffer releases previous map (if there is any) at very
beginning, so there is no problem even map_extent_buffer is called
multiple times.

Regards
YZ