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SubjectRe: [PATCH] mm: filemap: clear idle flag for writes
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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.


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