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

[Seth Jennings]

On Wed, Nov 19, 2014 at 09:06:41PM +0800, Weijie Yang wrote:
> On Wed, Nov 19, 2014 at 6:29 AM, Seth Jennings <sjennings@xxxxxxxxxxxxxx> wrote:
> > 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().
> 
> The frontswap_invalidate_page() is for swap_entry_free, while here
> is an inner ops for dup-store, so I think there is no need for
> inc_frontswap_invalidates().

In my mind, I agree we shouldn't call __frontswap_invalidate_page(),
just to keep things separated.

Andrew has already pulled it in and it isn't a big deal.  Just a
statistics thing on a rare situation (dup) counted along with lots
of frequent situations (normal invalidate).  Which makes me think
we make want to count dup-invalidates as a separate stat.  But that
would be a separate patch too :)

Thanks,
Seth

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