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

[Weijie Yang <weijie.yang.kh@gmail.com>]

On Wed, Nov 19, 2014 at 6:29 AM, Seth Jennings <sjennings@variantweb.net> 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@samsung.com>
>> ---
>>  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().

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