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

[Shaohua Li <shli@kernel.org>]

On Tue, Sep 26, 2017 at 03:40:17PM -0400, Johannes Weiner wrote:
> On Tue, Sep 26, 2017 at 10:26:26AM -0700, Shaohua Li wrote:
> > From: Shaohua Li <shli@fb.com>
> > 
> > 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?

That would work, but I don't see an easy/clean way to check the dirty bit.
Since the race is rare, I think this optimiztion isn't worthy.

Thanks,
Shaohua

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