linux-kernel - Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem

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Message-ID: <20190614062110.GF7246@lst.de>
Date:   Fri, 14 Jun 2019 08:21:10 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem_free

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..