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 <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Yu Zhao <[email protected]>
---
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
On Thu, May 24, 2018 at 5:57 AM, Li Wang <[email protected]> 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 <[email protected]>
> Cc: Seth Jennings <[email protected]>
> Cc: Dan Streetman <[email protected]>
> Cc: Huang Ying <[email protected]>
> Cc: Yu Zhao <[email protected]>
> ---
> 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
>
On Tue, May 29, 2018 at 10:57 PM, Li Wang <[email protected]> wrote:
> Hi Dan,
>
> On Wed, May 30, 2018 at 5:14 AM, Dan Streetman <[email protected]> wrote:
>>
>> On Thu, May 24, 2018 at 5:57 AM, Li Wang <[email protected]> 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