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Date: Fri, 1 May 2026 06:11:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Frederick Mayle <fmayle@google.com>
Cc: Matthew Wilcox <willy@infradead.org>, android-mm@google.com,
 kernel-team@android.com, Jan Kara <jack@suse.cz>,
 linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] mm/readahead: simplify page_cache_ra_unbounded loop
 counter reset
Message-Id: <20260501061146.6e61392d125cf1847d7cc181@linux-foundation.org>
In-Reply-To: <20260501011908.3630802-1-fmayle@google.com>
References: <20260501011908.3630802-1-fmayle@google.com>
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On Thu, 30 Apr 2026 18:19:07 -0700 Frederick Mayle <fmayle@google.com> wrote:

> Minor cleanup, no behavior change intended.
> 
> `read_pages` ensures that `ractl->_nr_pages` is zero before it returns,

So it seems, but depending upon this might be a bit fragile?

It would be better to make this a more explicit/formal part of the
read_pages() contract.  kerneldocifying read_pages() would be a
suitable way.

> so the `ractl->_nr_pages` term in these expressions contributes nothing.
> This seems to have been true since the statements were introduced in
> commit f615bd5c4725f ("mm/readahead: Handle ractl nr_pages being
> modified").
> 
> The new expression has an intuitive explanation. When filesystems
> perform readahead, they increment `ractl->_index` by the number of pages
> processed, so, after `read_pages` returns, `ractl->_index` points to the
> first page after those already processed. `index` points to the first
> page considered in the loop. So, `ractl->_index - index` is the number
> of pages processed by the loop so far.
>
> ...
>
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -270,7 +270,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  			 */
>  			read_pages(ractl);
>  			ractl->_index += min_nrpages;
> -			i = ractl->_index + ractl->_nr_pages - index;
> +			i = ractl->_index - index;
>  			continue;
>  		}
>  
> @@ -286,7 +286,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  				break;
>  			read_pages(ractl);
>  			ractl->_index += min_nrpages;
> -			i = ractl->_index + ractl->_nr_pages - index;
> +			i = ractl->_index - index;
>  			continue;
>  		}
>  		if (i == mark)
>