Comment on patch to remove nr_async_pages limit

[Marcelo Tosatti]

Zlatko, 

I've read your patch to remove nr_async_pages limit while reading an
archive on the web. (I have to figure out why lkml is not being delivered
correctly to me...)

Quoting your message: 

"That artificial limit hurts both swap out and swap in path as it
introduces synchronization points (and/or weakens swapin readahead),
which I think are not necessary."

If we are under low memory, we cannot simply writeout a whole bunch of
swap data. Remember the writeout operations will potentially allocate
buffer_head's for the swapcache pages before doing real IO, which takes
_more memory_: OOM deadlock.