linux-kernel - Re: [PATCH] mm: filemap: clear idle flag for writes

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Message-ID: <61fdaf3c-261f-9138-4c4e-a0a859094786@linux.alibaba.com>
Date:   Wed, 24 Jun 2020 13:24:08 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...riel.com>, gavin.dg@...ux.alibaba.com,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: filemap: clear idle flag for writes



On 6/24/20 12:50 PM, Shakeel Butt wrote:
> On Wed, Jun 24, 2020 at 12:18 PM Yang Shi <yang.shi@...ux.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@...ux.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.