From: Matthew Wilcox <willy@infradead.org>
To: Pasha Tatashin <pasha.tatashin@soleen.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
Subject: Re: [RFC 0/3] iommu/intel: Free empty page tables on unmaps
Date: Thu, 21 Dec 2023 04:16:34 +0000 [thread overview]
Message-ID: <ZYO8IqiHeqs8LktJ@casper.infradead.org> (raw)
In-Reply-To: <20231221031915.619337-1-pasha.tatashin@soleen.com>
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.
next prev parent reply other threads:[~2023-12-21 4:16 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-21 3:19 Pasha Tatashin
2023-12-21 3:19 ` [RFC 1/3] iommu/intel: Use page->refcount to count number of entries in IOMMU Pasha Tatashin
2023-12-25 2:59 ` Liu, Jingqi
2023-12-28 15:01 ` Pasha Tatashin
2023-12-21 3:19 ` [RFC 2/3] iommu/intel: synchronize page table map and unmap operations Pasha Tatashin
2023-12-21 3:19 ` [RFC 3/3] iommu/intel: free empty page tables on unmaps Pasha Tatashin
2023-12-21 4:16 ` Matthew Wilcox [this message]
2023-12-21 5:13 ` [RFC 0/3] iommu/intel: Free " Pasha Tatashin
2023-12-21 5:42 ` Pasha Tatashin
2023-12-21 14:06 ` Matthew Wilcox
2023-12-21 14:58 ` Pasha Tatashin
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