linux-kernel - Re: [PATCH v1] fs/dax: fix folio splitting issue by resetting old folio order + _nr

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Message-ID: <Z_g-Chjk12ijqf9O@casper.infradead.org>
Date: Thu, 10 Apr 2025 22:54:18 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Dan Williams <dan.j.williams@...el.com>
Cc: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	nvdimm@...ts.linux.dev,
	Alison Schofield <alison.schofield@...el.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alistair Popple <apopple@...dia.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v1] fs/dax: fix folio splitting issue by resetting old
 folio order + _nr_pages

On Thu, Apr 10, 2025 at 01:46:06PM -0700, Dan Williams wrote:
> Matthew Wilcox wrote:
> > On Thu, Apr 10, 2025 at 01:15:07PM -0700, Dan Williams wrote:
> > > For consistency and clarity what about this incremental change, to make
> > > the __split_folio_to_order() path reuse folio_reset_order(), and use
> > > typical bitfield helpers for manipulating _flags_1?
> > 
> > I dislike this intensely.  It obfuscates rather than providing clarity.
> 
> I'm used to pushing folks to use bitfield.h in driver land, but will not
> push it further here.

I think it can make sense in places.  Just not here.

> What about this hunk?
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2a47682d1ab7..301ca9459122 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3404,7 +3404,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  	if (new_order)
>  		folio_set_order(folio, new_order);
>  	else
> -		ClearPageCompound(&folio->page);
> +		folio_reset_order(folio);
>  }

I think that's wrong.  We're splitting this folio into order-0 folios,
but folio_reset_order() is going to modify folio->_flags_1 which is in
the next page.