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

[Li Wang <liwang@redhat.com>]

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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?

-- 
Regards,
Li Wang

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