On Tue, Sep 26, 2017 at 10:26:26AM -0700, Shaohua Li wrote:

From: Shaohua Li <redacted>

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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