linux-kernel - Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps

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Date: Fri, 26 Apr 2024 09:49:23 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rientjes@...gle.com, dwmw2@...radead.org, 
	baolu.lu@...ux.intel.com, joro@...tes.org, will@...nel.org, 
	robin.murphy@....com, iommu@...ts.linux.dev, 
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps

On Fri, Apr 26, 2024 at 2:42 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 26.04.24 05:43, Pasha Tatashin wrote:
> > Changelog
> > ================================================================
> > v2: Use mapcount instead of refcount
> >      Synchronized with IOMMU Observability changes.
> > ================================================================
> >
> > 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