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

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Message-Id: <20200624115317.792d8fc6369d421d2898ab2f@linux-foundation.org>
Date:   Wed, 24 Jun 2020 11:53:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     hannes@...xchg.org, riel@...riel.com, shakeelb@...gle.com,
        gavin.dg@...ux.alibaba.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: filemap: clear idle flag for writes

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.

This change seems correct but also will have runtime effects.  What are
they?