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SubjectRe: Race in shrink_cache
Date
On Thursday 05 September 2002 09:07, Andrew Morton wrote:
> Daniel Phillips wrote:
> >
> > ...
> > /*
> > * We must not allow an anon page
> > * with no buffers to be visible on
> > * the LRU, so we unlock the page after
> > * taking the lru lock
> > */
> >
> > That is, what's scary about an anon page without buffers?
>
> 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.

With the atomic_dec_and_lock strategy, the page would be freed immediately on
the buffers being released, and with the lru=1 strategy it doesn't matter
in terms of correctness whether the page ends up on the lru or not, so I was
inclined not to worry about this anyway, still, when you a dire-looking
comment like that...

You said something about your lru locking strategy in 2.5.33-mm2. I have not
reverse engineered it yet, would you care to wax poetic?

--
Daniel
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