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

[Dan Streetman <ddstreet@ieee.org>]

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.

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).

>
> Signed-off-by: Li Wang <liwang@redhat.com>
> Cc: Seth Jennings <sjenning@redhat.com>
> Cc: Dan Streetman <ddstreet@ieee.org>
> Cc: Huang Ying <huang.ying.caritas@gmail.com>
> Cc: Yu Zhao <yuzhao@google.com>
> ---
>  mm/zswap.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 61a5c41..2b537bb 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1007,6 +1007,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         u8 *src, *dst;
>         struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
>
> +       if (!zswap_max_pool_percent) {
> +               ret = -ENOMEM;
> +               goto reject;
> +       }
> +
>         /* THP isn't supported */
>         if (PageTransHuge(page)) {
>                 ret = -EINVAL;
> --
> 2.9.5
>