linux-kernel - Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio

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Message-ID: <20260112192816.GL745888@ziepe.ca>
Date: Mon, 12 Jan 2026 15:28:16 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Zi Yan <ziy@...dia.com>
Cc: Matthew Wilcox <willy@...radead.org>, Balbir Singh <balbirs@...dia.com>,
	Francois Dugast <francois.dugast@...el.com>,
	intel-xe@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
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Subject: Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio_free
 callback

On Mon, Jan 12, 2026 at 01:55:18PM -0500, Zi Yan wrote:
> > That's different, I am talking about reaching 0 because it has been
> > freed, meaning there are no external pointers to it.
> >
> > Further, when a page is frozen page_ref_freeze() takes in the number
> > of references the caller has ownership over and it doesn't succeed if
> > there are stray references elsewhere.
> >
> > This is very important because the entire operating model of split
> > only works if it has exclusive locks over all the valid pointers into
> > that page.
> >
> > Spurious refcount failures concurrent with split cannot be allowed.
> >
> > I don't see how pointing at __folio_freeze_and_split_unmapped() can
> > justify this series.
> >
> 
> But from anyone looking at the folio state, refcount == 0, compound_head
> is set, they cannot tell the difference.

This isn't reliable, nothing correct can be doing it :\

> If what you said is true, why is free_pages_prepare() needed? No one
> should touch these free pages. Why bother resetting these states.

? that function does alot of stuff, thinks like uncharging the cgroup
should obviously happen at free time.

What part of it are you looking at?

> > You can't refcount a folio out of nothing. It has to come from a
> > memory location that already is holding a refcount, and then you can
> > incr it.
> 
> Right. There is also no guarantee that all code is correct and follows
> this.

Let's concretely point at things that have a problem please.

> My point here is that calling prep_compound_page() on a compound page
> does not follow core MM’s conventions.

Maybe, but that doesn't mean it isn't the right solution..

Jason