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SubjectRe: VM: Fix nasty and subtle race in shared mmap'ed page writeback


On Thu, 4 Jan 2007, Nick Piggin wrote:
>
> Yhat's when the bug was introduced -- 2.6.19. 2.6.18 does not have
> this bug, so it cannot be years old.

Actually, I think 2.6.18 may have a subtle variation on it.

In particular, I look back at the try_to_free_buffers() thing that I hated
so much, and it makes me wonder.. It used to do:

spin_lock(&mapping->private_lock);
ret = drop_buffers(page, &buffers_to_free);
spin_unlock(&mapping->private_lock);
if (ret) {
.. crappy comment ..
if (test_clear_page_dirty(page))
task_io_account_cancelled_write(PAGE_CACHE_SIZE);
}

and I think that at least on SMP, we had a race with another CPU doing the
"mark page dirty if it was dirty in the PTE" at the same time. Because the
marking dirty would come in, find no buffers (they just got dropped), and
then mark the page dirty (ignoring the lack of any buffers), but then the
above would do the "test_clear_page_dirty()" thing on it.

Ie the race, I think, existed where that crappy comment was.

But that much older race would only trigger on SMP (or possibly UP with
preempt).

And I haven't actually thought about it that much, so I could be full of
crap. But I don't see anything that protects against it: we may hold the
page lock, but since the code that marks things _dirty_ doesn't
necessarily always hold it, that doesn't help us. And we may hold the
"private_lock", but we drop it before we do the dirty bit clearing, and in
fact on UP+PREEMPT that very dropping could cause an active preemption to
take place, so..

I dunno.

I would certainly suggest the whole series of my "dirty cleanup" be added
on top of 2.6.18.3 (which apparently has the dirty mapped page tracking).
For older kernels? If there is a race there, it must be pretty damn hard
to hit in practice (and it must have been there for a looong time), so
trying to fix it is possibly as likely to cause problems as it migh to fix
them.

Linus
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