Re: [PATCH] page->flags corruption fix

[Hugh Dickins]

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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