Re: [PATCH V3 2/2] mm: fix data corruption caused by lazyfree page

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On Tue, Sep 26, 2017 at 10:26:26AM -0700, Shaohua Li wrote:
> From: Shaohua Li <shli@xxxxxx>
> 
> MADV_FREE clears pte dirty bit and then marks the page lazyfree (clear
> SwapBacked). There is no lock to prevent the page is added to swap cache
> between these two steps by page reclaim. If page reclaim finds such
> page, it will simply add the page to swap cache without pageout the page
> to swap because the page is marked as clean. Next time, page fault will
> read data from the swap slot which doesn't have the original data, so we
> have a data corruption. To fix issue, we mark the page dirty and pageout
> the page.

Reclaim and MADV_FREE hold the page lock when manipulating the dirty
and the swapcache state.

Instead of undoing a racing MADV_FREE in reclaim, wouldn't it be safe
to check the dirty bit before add_to_swap() and skip clean pages?



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