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


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