Messages in this thread | | | Date | Thu, 04 Jan 2007 16:07:18 +1100 | From | Nick Piggin <> | Subject | Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback |
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Linus Torvalds wrote: > > 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).
Oh yes the try_to_free_buffers race, I think, does exist in older kernels. Yes according to our earlier analysis it would trigger with UP+preempt and SMP.
But the patch that Andrea was pointing to was your last patch (The Fix), which stopped page_mkclean caller throwing out dirty bits. You probably didn't see that in the mail I cc'ed you on.
So yes it would be interesting to see whether fixing try_to_free_buffers fixes Andrea's problem on older kernels.
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