linux-kernel - Re: [PATCH] zsmalloc: zs_page_migrate: schedule free_work if zspage is ZS

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Date:   Mon, 14 Aug 2017 17:11:50 +0800
From:   Hui Zhu <teawater@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Hui Zhu <zhuhui@...omi.com>,
        "ngupta@...are.org" <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zsmalloc: zs_page_migrate: schedule free_work if zspage
 is ZS_EMPTY

2017-08-14 16:31 GMT+08:00 Minchan Kim <minchan@...nel.org>:
> Hi Hui,
>
> On Mon, Aug 14, 2017 at 02:34:46PM +0800, Hui Zhu wrote:
>> After commit e2846124f9a2 ("zsmalloc: zs_page_migrate: skip unnecessary
>> loops but not return -EBUSY if zspage is not inuse") zs_page_migrate
>> can handle the ZS_EMPTY zspage.
>>
>> But it will affect the free_work free the zspage.  That will make this
>> ZS_EMPTY zspage stay in system until another zspage wake up free_work.
>>
>> Make this patch let zs_page_migrate wake up free_work if need.
>>
>> Fixes: e2846124f9a2 ("zsmalloc: zs_page_migrate: skip unnecessary loops but not return -EBUSY if zspage is not inuse")
>> Signed-off-by: Hui Zhu <zhuhui@...omi.com>
>
> This patch makes me remind why I didn't try to migrate empty zspage
> as you did e2846124f9a2. I have forgotten it toally.
>
> We cannot guarantee when the freeing of the page happens if we use
> deferred freeing in zs_page_migrate. However, we returns
> MIGRATEPAGE_SUCCESS which is totally lie.
> Without instant freeing the page, it doesn't help the migration
> situation. No?
>

Sorry I think the reason is I didn't introduce this clear.
After I patch e2846124f9a2.  I got some false in zs_page_isolate:
if (get_zspage_inuse(zspage) == 0) {
spin_unlock(&class->lock);
return false;
}
The page of this zspage was migrated in before.

So I think e2846124f9a2 is OK that MIGRATEPAGE_SUCCESS with the "page".
But it keep the "newpage" with a empty zspage inside system.
Root cause is zs_page_isolate remove it from  ZS_EMPTY list but not
call zs_page_putback "schedule_work(&pool->free_work);".  Because
zs_page_migrate done the job without
"schedule_work(&pool->free_work);"

That is why I made the new patch.

Thanks,
Hui

> I start to wonder why your patch e2846124f9a2 helped your test.
> I will think over the issue with fresh mind after the holiday.
>
>> ---
>>  mm/zsmalloc.c | 10 ++++++++--
>>  1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> index 62457eb..48ce043 100644
>> --- a/mm/zsmalloc.c
>> +++ b/mm/zsmalloc.c
>> @@ -2035,8 +2035,14 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>>        * Page migration is done so let's putback isolated zspage to
>>        * the list if @page is final isolated subpage in the zspage.
>>        */
>> -     if (!is_zspage_isolated(zspage))
>> -             putback_zspage(class, zspage);
>> +     if (!is_zspage_isolated(zspage)) {
>> +             /*
>> +              * The page and class is locked, we cannot free zspage
>> +              * immediately so let's defer.
>> +              */
>> +             if (putback_zspage(class, zspage) == ZS_EMPTY)
>> +                     schedule_work(&pool->free_work);
>> +     }
>>
>>       reset_page(page);
>>       put_page(page);
>> --
>> 1.9.1
>>
>> --
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