Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] mm: filemap: clear idle flag for writes
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Rik van Riel <riel@surriel.com>, gavin.dg@linux.alibaba.com,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <1593020612-13051-1-git-send-email-yang.shi@linux.alibaba.com>
 <20200624115317.792d8fc6369d421d2898ab2f@linux-foundation.org>
 <b1959a9b-d660-3a99-df37-fea141eda44d@linux.alibaba.com>
 <CALvZod6BEw-1=XgwN6RDohnEp1uwmDQwPPvgbtC2H4q-X=Nsig@mail.gmail.com>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <61fdaf3c-261f-9138-4c4e-a0a859094786@linux.alibaba.com>
Date:   Wed, 24 Jun 2020 13:24:08 -0700
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0)
 Gecko/20100101 Thunderbird/52.7.0
MIME-Version: 1.0
In-Reply-To: <CALvZod6BEw-1=XgwN6RDohnEp1uwmDQwPPvgbtC2H4q-X=Nsig@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 6/24/20 12:50 PM, Shakeel Butt wrote:
> On Wed, Jun 24, 2020 at 12:18 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>
>>
>> On 6/24/20 11:53 AM, Andrew Morton wrote:
>>> On Thu, 25 Jun 2020 01:43:32 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>
>>>> Since commit bbddabe2e436aa7869b3ac5248df5c14ddde0cbf ("mm: filemap:
>>>> only do access activations on reads"), mark_page_accessed() is called
>>>> for reads only.  But the idle flag is cleared by mark_page_accessed() so
>>>> the idle flag won't get cleared if the page is write accessed only.
>>>>
>>>> Basically idle page tracking is used to estimate workingset size of
>>>> workload, noticeable size of workingset might be missed if the idle flag
>>>> is not maintained correctly.
>>>>
>>>> It seems good enough to just clear idle flag for write operations.
>>>>
>>>> ...
>>>>
>>>> --- a/mm/filemap.c
>>>> +++ b/mm/filemap.c
>>>> @@ -41,6 +41,7 @@
>>>>    #include <linux/delayacct.h>
>>>>    #include <linux/psi.h>
>>>>    #include <linux/ramfs.h>
>>>> +#include <linux/page_idle.h>
>>>>    #include "internal.h"
>>>>
>>>>    #define CREATE_TRACE_POINTS
>>>> @@ -1630,6 +1631,11 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
>>>>
>>>>       if (fgp_flags & FGP_ACCESSED)
>>>>               mark_page_accessed(page);
>>>> +    else if (fgp_flags & FGP_WRITE) {
>>>> +            /* Clear idle flag for buffer write */
>>>> +            if (page_is_idle(page))
>>>> +                    clear_page_idle(page);
>>>> +    }
>>>>
>>>>    no_page:
>>>>       if (!page && (fgp_flags & FGP_CREAT)) {
>>> The kerneldoc comment for pagecache_get_page() could do with some
>>> updating - it fails to mention FGP_WRITE, FGP_NOFS and FGP_NOWAIT.
>> Yes, will propose a separate patch later on.
>>
>>> This change seems correct but also will have runtime effects.  What are
>>> they?
>> Other than a couple of extra cycles when idle page tracking is enabled,
>> I didn't think of other effects. It should be negligible. The idle flag
>> doesn't play a role in page reclaim algorithm, so it won't have impact
>> on that.
>>
>>
> The only user visible impact will be on idle page tracking users. They
> will get more accurate data.

Thanks for elaborating this.