ooop. That's an akpm comment. umm, err..
It solves this BUG:
http://www.cs.helsinki.fi/linux/linux-kernel/2001-37/0594.html
Around the 2.4.10 timeframe, Andrea started putting anon pages
on the LRU. Then he backed that out, then put it in again. I
think this comment dates from the time when anon pages were
not on the LRU. So there's a little window there where the
page is unlocked, we've just dropped its swapdev buffers, the page is
on the LRU and pagemap_lru_lock is not held.
So another CPU came in, found the page on the LRU, saw that it had
no ->mapping and no ->buffers and went BUG.
The fix was to take pagemap_lru_lock before unlocking the page.
The comment is stale.
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