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From: Pasha Tatashin <pasha.tatashin@soleen.com>
Date: Fri, 26 Apr 2024 09:49:23 -0400
Message-ID: <CA+CK2bC4SWTCG2bnA16Xe+gX7=N=UYWB1wSns-K-jNqC1yrdvQ@mail.gmail.com>
Subject: Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps
To: David Hildenbrand <david@redhat.com>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, rientjes@google.com, dwmw2@infradead.org, 
	baolu.lu@linux.intel.com, joro@8bytes.org, will@kernel.org, 
	robin.murphy@arm.com, iommu@lists.linux.dev, 
	Matthew Wilcox <willy@infradead.org>
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On Fri, Apr 26, 2024 at 2:42=E2=80=AFAM David Hildenbrand <david@redhat.com=
> wrote:
>
> On 26.04.24 05:43, Pasha Tatashin wrote:
> > Changelog
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> > v2: Use mapcount instead of refcount
> >      Synchronized with IOMMU Observability changes.
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >
> > This series frees empty page tables on unmaps. It intends to be a
> > low overhead feature.
> >
> > The read-writer lock is used to synchronize page table, but most of
> > time the lock is held is reader. It is held as a writer for short
> > period of time when unmapping a page that is bigger than the current
> > iova request. For all other cases this lock is read-only.
> >
> > page->mapcount is used in order to track number of entries at each page
> > table.
>
> I'm wondering if this will conflict with page_type at some point? We're
> already converting other page table users to ptdesc. CCing Willy.

Hi David,

This contradicts with the following comment in mm_types.h:
 * If your page will not be mapped to userspace, you can also use the four
 * bytes in the mapcount union, but you must call
page_mapcount_reset()
 * before freeing it.

Thank you,
Pasha