linux-kernel - Re: [PATCH v7 04/11] readahead: allocate folios with mapping_min

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Date: Wed, 12 Jun 2024 19:50:49 +0100
From: Matthew Wilcox <willy@...radead.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: david@...morbit.com, djwong@...nel.org, chandan.babu@...cle.com,
	brauner@...nel.org, akpm@...ux-foundation.org, mcgrof@...nel.org,
	linux-mm@...ck.org, hare@...e.de, linux-kernel@...r.kernel.org,
	yang@...amperecomputing.com, Zi Yan <zi.yan@...t.com>,
	linux-xfs@...r.kernel.org, p.raghav@...sung.com,
	linux-fsdevel@...r.kernel.org, hch@....de, gost.dev@...sung.com,
	cl@...amperecomputing.com, john.g.garry@...cle.com
Subject: Re: [PATCH v7 04/11] readahead: allocate folios with
 mapping_min_order in readahead

On Fri, Jun 07, 2024 at 02:58:55PM +0000, Pankaj Raghav (Samsung) wrote:
> @@ -230,7 +247,9 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  		struct folio *folio = xa_load(&mapping->i_pages, index + i);
>  		int ret;
>  
> +

Spurious newline

>  		if (folio && !xa_is_value(folio)) {
> +			long nr_pages = folio_nr_pages(folio);

Hm, but we don't have a reference on this folio.  So this isn't safe.

> @@ -240,12 +259,24 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>  			 * not worth getting one just for that.
>  			 */
>  			read_pages(ractl);
> -			ractl->_index += folio_nr_pages(folio);
> +
> +			/*
> +			 * Move the ractl->_index by at least min_pages
> +			 * if the folio got truncated to respect the
> +			 * alignment constraint in the page cache.
> +			 *
> +			 */
> +			if (mapping != folio->mapping)
> +				nr_pages = min_nrpages;
> +
> +			VM_BUG_ON_FOLIO(nr_pages < min_nrpages, folio);
> +			ractl->_index += nr_pages;

Why not just:
			ractl->_index += min_nrpages;

like you do below?