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

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Date: Thu, 21 Dec 2023 04:16:34 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Pasha Tatashin <pasha.tatashin@...een.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
Subject: Re: [RFC 0/3] iommu/intel: Free empty page tables on unmaps

On Thu, Dec 21, 2023 at 03:19:12AM +0000, Pasha Tatashin wrote:
> 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->refcount is used in order to track number of entries at each page
> table.

Have I not put enough DANGER signs up around the page refcount?

 * If you want to use the refcount field, it must be used in such a way
 * that other CPUs temporarily incrementing and then decrementing the
 * refcount does not cause problems.  On receiving the page from
 * alloc_pages(), the refcount will be positive.

You can't use refcount for your purpose, and honestly I'm shocked you
haven't seen any of your WARNings trigger.