linux-kernel - Re: [PATCH] dma-direct: fix the page free when it is not addressable

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Message-ID: <20240903073521.GA2269@lst.de>
Date: Tue, 3 Sep 2024 09:35:21 +0200
From: Christoph Hellwig <hch@....de>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>, Baolu Lu <baolu.lu@...el.com>,
	Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: fix the page free when it is not
 addressable

On Sat, Aug 31, 2024 at 07:01:19PM +0800, Chen Yu wrote:
> When the CMA allocation succeeds but isn't addressable, its
> buffer has already been released and the page is set to NULL.
> So later when the normal page allocation succeeds but isn't
> addressable, __free_pages() should be used to free that normal
> page rather than freeing continuous page(CMA).

So the patch is obviously correct and probably useful, but I don't think
the existing code is buggy.

dma_free_contiguous calls into cma_release, which uses cma_pages_valid to
check if the page belongs to the CMA pool and retuns early if not,
letting dma_free_contiguous fall back to __free_pages eventually.

What am I missing here?