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Date: Fri, 14 Jun 2019 08:21:10 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem_free
Message-ID: <20190614062110.GF7246@lst.de>
References: <20190613094326.24093-1-hch@lst.de>
 <20190613094326.24093-5-hch@lst.de> <20190613190501.GQ22062@mellanox.com>
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To: Jason Gunthorpe <jgg@mellanox.com>
Cc: "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>, "nouveau@lists.freedesktop.org" <nouveau@lists.freedesktop.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>, Ben Skeggs <bskeggs@redhat.com>, "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>, Christoph Hellwig <hch@lst.de>
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On Thu, Jun 13, 2019 at 07:05:07PM +0000, Jason Gunthorpe wrote:
> Hurm, is hmm following this comment from mm_types.h?
> 
>  * If you allocate the page using alloc_pages(), you can use some of the
>  * space in struct page for your own purposes.  The five words in the main
>  * union are available, except for bit 0 of the first word which must be
>  * kept clear.  Many users use this word to store a pointer to an object
>  * which is guaranteed to be aligned.  If you use the same storage as
>  * page->mapping, you must restore it to NULL before freeing the page.
> 
> Maybe the assumption was that a driver is using ->mapping ?

Maybe.  The union layou in struct page certainly doesn't help..
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