Hi All,
I have noticed that, reading or dumping a full sysfs entry (ex: 
/sys/kernel/debug/gpio )
which exceeds PAGE_SIZE restarts associated seq_file iterator (calling 
start->show->stop)
morethan one time.

In my case gpio entry just exceeded 8k which resulted in show function 
getting
called 3 times.

The reason is because the allocated buffer overflows and seq_file allocates
new buffer of next order and starts iteration from the start, which is 
clear in
seq_file code.
However this is bit of conservative approach which results in:
- restarting iterator if file sz exceeds PAGE_SIZE( which is really not 
necessary )
- not reusing the formated buffer (which can be done by re-alloc and copy).
- Will help entires like dumping cacheline or pagetables to capture its 
state with better accuracy.

To address this situation, I think seq_file formating/printf functions 
should
re-allocate buffer as soon as they detect buffer overflow.

This patch attempts to do the same by adding new function buf_realloc 
which inturn
calls krealloc and all the seq_file formating or printing functions call 
this
function whenever an overflow is detected and retry.


Main motive of this patch is to avoid restarting the iterator and NOT
performance, however there is a bit of gain in performance too.

Performance figures(average over 100 times loop) for different sizes of
sysfs entries:

without realloc:
---------------------------------
   SZ        REAL    USER    SYS
---------------------------------
| 4k   | 0.012    0.002    0.009
| 8k   | 0.013    0.002    0.010
| 16k  | 0.015    0.003    0.012
| 32k  | 0.019    0.002    0.017
--------------------------------

with realloc:
---------------------------------
   SZ        REAL    USER    SYS
---------------------------------
| 4k  |    0.012    0.002    0.009
| 8k  |    0.012    0.002    0.009
| 16k |    0.013    0.002    0.010
| 32k |    0.015    0.003    0.012
-------------------------------


I discovered this issue while I was working on a different issue as I 
happed
to stick a printk in show function, which got called 3 times when I 
dumped the
entry with 'cat' :-). which I did not expect.


comments? :-)

--srini