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

[Li Wang <liwang@xxxxxxxxxx>]

Hi Dan,

On Wed, May 30, 2018 at 5:14 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:

On Thu, May 24, 2018 at 5:57 AM, Li Wang <liwang@xxxxxxxxxx> 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?

--

Regards,

Li Wang