Messages in this thread | | | Date | Wed, 8 Oct 2003 18:15:08 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [PATCH] page->flags corruption fix |
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On Wed, 8 Oct 2003, Rik van Riel wrote: > > 1) cpu A adds page P to the swap cache, loading page->flags > and modifying it locally
Right, the add_to_swap_cache in try_to_swap_out.
> 2) a second thread scans a page table entry and sees that > the page was accessed, so cpu B moves page P to the > active list
Right, the mark_page_accessed in try_to_swap_out (I don't see any other mark_page_accessed as problematic). Or the del_page_from_active_list in refill_inactive.
> 3) cpu A undoes the PG_inactive -> PG_active bit change, > corrupting the page->flags of P
(Mainline is easier than -rmap since no separate PG_inactive bit.)
Thanks a lot for explaining, I see it now. Personally I'd prefer a lighter weight patch, either pagemap_lru_lock within add_to_swap_cache, or better moving the PG_flags clearing from __add_to_page_cache into __free_pages_ok, where I still believe it can be done non-atomically.
But you've proved your point, and I understand you preferring a more straightforward and future-proof way. I'm not writing an alternative patch, don't let me stand in the way of progress...
A little of the above explanation in the change comment would be nice.
Thanks, Hugh
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