Re: [PATCH RFC] zswap: reject to compress/store page if zswap_max_pool_percent is 0

[Dan Streetman <ddstreet@ieee.org>]

On Tue, May 29, 2018 at 10:57 PM, Li Wang <liwang@redhat.com> wrote:
> Hi Dan,
>
> On Wed, May 30, 2018 at 5:14 AM, Dan Streetman <ddstreet@ieee.org> wrote:
>>
>> On Thu, May 24, 2018 at 5:57 AM, Li Wang <liwang@redhat.com> wrote:
>> > The '/sys/../zswap/stored_pages:' keep raising in zswap test with
>> > "zswap.max_pool_percent=0" parameter. But theoretically, it should
>> > not compress or store pages any more since there is no space for
>> > compressed pool.
>> >
>> > Reproduce steps:
>> >
>> >   1. Boot kernel with "zswap.enabled=1 zswap.max_pool_percent=17"
>> >   2. Set the max_pool_percent to 0
>> >       # echo 0 > /sys/module/zswap/parameters/max_pool_percent
>> >      Confirm this parameter works fine
>> >       # cat /sys/kernel/debug/zswap/pool_total_size
>> >       0
>> >   3. Do memory stress test to see if some pages have been compressed
>> >       # stress --vm 1 --vm-bytes $mem_available"M" --timeout 60s
>> >      Watching the 'stored_pages' numbers increasing or not
>> >
>> > The root cause is:
>> >
>> >   When the zswap_max_pool_percent is set to 0 via kernel parameter, the
>> > zswap_is_full()
>> >   will always return true to shrink the pool size by zswap_shrink(). If
>> > the pool size
>> >   has been shrinked a little success, zswap will do compress/store pages
>> > again. Then we
>> >   get fails on that as above.
>>
>> special casing 0% doesn't make a lot of sense to me, and I'm not
>> entirely sure what exactly you are trying to fix here.
>
>
> Sorry for that confusing, I am a pretty new to zswap.
>
> To specify 0 to max_pool_percent is purpose to verify if zswap stopping work
> when there is no space in compressed pool.
>
> Another consideration from me is:
>
> [Method A]
>
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1021,7 +1021,7 @@ static int zswap_frontswap_store(unsigned type,
> pgoff_t offset,
>         /* reclaim space if needed */
>         if (zswap_is_full()) {
>                 zswap_pool_limit_hit++;
> -               if (zswap_shrink()) {
> +               if (!zswap_max_pool_percent || zswap_shrink()) {
>                         zswap_reject_reclaim_fail++;
>                         ret = -ENOMEM;
>                         goto reject;
>
> This make sure the compressed pool is enough to do zswap_shrink().
>
>
>>
>>
>> however, zswap does currently do a zswap_is_full() check, and then if
>> it's able to reclaim a page happily proceeds to store another page,
>> without re-checking zswap_is_full().  If you're trying to fix that,
>> then I would ack a patch that adds a second zswap_is_full() check
>> after zswap_shrink() to make sure it's now under the max_pool_percent
>> (or somehow otherwise fixes that behavior).
>>
>
> Ok, it sounds like can also fix the issue. The changes maybe like:
>
> [Method B]
>
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1026,6 +1026,15 @@ static int zswap_frontswap_store(unsigned type,
> pgoff_t offset,
>                         ret = -ENOMEM;
>                         goto reject;
>                 }
> +
> +               /* A second zswap_is_full() check after
> +                * zswap_shrink() to make sure it's now
> +                * under the max_pool_percent
> +                */
> +               if (zswap_is_full()) {
> +                       ret = -ENOMEM;
> +                       goto reject;
> +               }
>         }
>
>
> So, which one do you think is better, A or B?

this is better.

>
> --
> Regards,
> Li Wang