linux-kernel - Re: [PATCH v6 03/11] filemap: allocate mapping_min

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Date: Mon, 3 Jun 2024 13:18:45 +0100
From: Matthew Wilcox <willy@...radead.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: david@...morbit.com, chandan.babu@...cle.com, akpm@...ux-foundation.org,
	brauner@...nel.org, djwong@...nel.org, linux-kernel@...r.kernel.org,
	hare@...e.de, john.g.garry@...cle.com, gost.dev@...sung.com,
	yang@...amperecomputing.com, p.raghav@...sung.com,
	cl@...amperecomputing.com, linux-xfs@...r.kernel.org, hch@....de,
	mcgrof@...nel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 03/11] filemap: allocate mapping_min_order folios in
 the page cache

On Wed, May 29, 2024 at 03:45:01PM +0200, Pankaj Raghav (Samsung) wrote:
> @@ -1919,8 +1921,10 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  		folio_wait_stable(folio);
>  no_page:
>  	if (!folio && (fgp_flags & FGP_CREAT)) {
> -		unsigned order = FGF_GET_ORDER(fgp_flags);
> +		unsigned int min_order = mapping_min_folio_order(mapping);
> +		unsigned int order = max(min_order, FGF_GET_ORDER(fgp_flags));
>  		int err;
> +		index = mapping_align_start_index(mapping, index);
>  
>  		if ((fgp_flags & FGP_WRITE) && mapping_can_writeback(mapping))
>  			gfp |= __GFP_WRITE;
> @@ -1958,7 +1962,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  				break;
>  			folio_put(folio);
>  			folio = NULL;
> -		} while (order-- > 0);
> +		} while (order-- > min_order);

I'd argue you also need to change:

-                       if (order > 0)
+			if (order > min_order)
                                alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;

since that is the last point at which we can fall back.  If we can't
immediately allocate a min_order folio, we want to retry, and we
want to warn if we can't get it.