linux-kernel - Re: [PATCH 09/16] page-flags: define PG

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Date:   Mon, 3 Feb 2020 09:29:09 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...two.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Steve Capper <steve.capper@...aro.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.cz>,
        Jerome Marchand <jmarchan@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 09/16] page-flags: define PG_reserved behavior on
 compound pages

On Mon, Feb 03, 2020 at 06:18:44PM +0300, Kirill A. Shutemov wrote:
> > Much later than you would ever expect, but we just had a user update an
> > ancient device and trip over this.
> > https://gitlab.freedesktop.org/drm/intel/issues/1027
> > 
> > In drm_pci_alloc() we allocate a high-order page (for it to be physically
> > contiguous) and mark each page as Reserved.
> > 
> >         dmah->vaddr = dma_alloc_coherent(&dev->pdev->dev, size,
> >                                          &dmah->busaddr,
> >                                          GFP_KERNEL | __GFP_COMP);
> > 
> >         /* XXX - Is virt_to_page() legal for consistent mem? */
> >         /* Reserve */
> >         for (addr = (unsigned long)dmah->vaddr, sz = size;
> >              sz > 0; addr += PAGE_SIZE, sz -= PAGE_SIZE) {
> >                 SetPageReserved(virt_to_page((void *)addr));
> >         }
> > 
> > It's been doing that since

This code is completely and utterly broken.  Drivers were never allowed
to call virt_to_page() on the memory returned from dma_alloc_coherent
(or pci_alloc_consistent before that), as many implementations return
virtual addresses that are not in the kernel mapping.  So this code
needs to go away and not papered over.