linux-kernel - Re: [PATCH 3/3] mm/readahead: try to allocate high order pages for FADVISE_FAV

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Message-ID: <aTCJ0Js4X7qlqzDZ@google.com>
Date: Wed, 3 Dec 2025 19:04:48 +0000
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
	linux-mm@...ck.org
Subject: Re: [PATCH 3/3] mm/readahead: try to allocate high order pages for
 FADVISE_FAV_WILLNEED

On 12/02, Matthew Wilcox wrote:
> On Tue, Dec 02, 2025 at 01:30:13AM +0000, Jaegeuk Kim wrote:
> > @@ -627,7 +628,7 @@ void page_cache_sync_ra(struct readahead_control *ractl,
> >  	ra->size = min(contig_count + req_count, max_pages);
> >  	ra->async_size = 1;
> >  readit:
> > -	ra->order = 0;
> > +	ra->order = mapping_max_folio_order(ractl->mapping);
> >  	ractl->_index = ra->start;
> >  	page_cache_ra_order(ractl, ra);
> >  }
> 
> I suspect this is in the wrong place, but I'm on holiday and not going
> to go spelunking through the readahead code looking for the right place.
> 
> Also, going directly to max folio order is wrong, we should use the same
> approach as the write order code, encapsulated in filemap_get_order().
> See 4f6617011910

It seems the key is page_cache_ra_order() which allocates pages by
ra_alloc_folio() given ra->order. FWIW, madvise() and fault() readahead
takes page_cache_async_ra(), while fadvise() takes page_cache_sync_ra().
And, the former one has a logic to bump up the ra->order += 2 by f838ddf8cef5.
I think it'd make sense to match that behavior?