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

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

[Li Wang]

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.

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

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

[Dan Streetman]

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
>

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

[Li Wang]

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

[Dan Streetman]

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