Re: [PATCH] mm: frontswap: invalidate expired data on a dup-store failure

[Seth Jennings]

On Tue, Nov 18, 2014 at 04:51:36PM +0800, Weijie Yang wrote:
> If a frontswap dup-store failed, it should invalidate the expired page
> in the backend, or it could trigger some data corruption issue.
> Such as:
> 1. use zswap as the frontswap backend with writeback feature
> 2. store a swap page(version_1) to entry A, success
> 3. dup-store a newer page(version_2) to the same entry A, fail
> 4. use __swap_writepage() write version_2 page to swapfile, success
> 5. zswap do shrink, writeback version_1 page to swapfile
> 6. version_2 page is overwrited by version_1, data corrupt.

Good catch!

> 
> This patch fixes this issue by invalidating expired data immediately
> when meet a dup-store failure.
> 
> Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
> ---
>  mm/frontswap.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/frontswap.c b/mm/frontswap.c
> index c30eec5..f2a3571 100644
> --- a/mm/frontswap.c
> +++ b/mm/frontswap.c
> @@ -244,8 +244,10 @@ int __frontswap_store(struct page *page)
>  		  the (older) page from frontswap
>  		 */
>  		inc_frontswap_failed_stores();
> -		if (dup)
> +		if (dup) {
>  			__frontswap_clear(sis, offset);
> +			frontswap_ops->invalidate_page(type, offset);

Looking at __frontswap_invalidate_page(), should we do
inc_frontswap_invalidates() too?  If so, maybe we should just call
__frontswap_invalidate_page().

Thanks,
Seth

> +		}
>  	}
>  	if (frontswap_writethrough_enabled)
>  		/* report failure so swap also writes to swap device */
> -- 
> 1.7.0.4
> 
> 
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