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

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Message-ID: <158048425224.2430.4905670949721797624@skylake-alporthouse-com>
Date:   Fri, 31 Jan 2020 15:24:12 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>
Cc:     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,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 09/16] page-flags: define PG_reserved behavior on compound pages

Quoting Kirill A. Shutemov (2015-03-19 17:08:15)
> As far as I can see there's no users of PG_reserved on compound pages.
> Let's use NO_COMPOUND here.

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

commit ddf19b973be5a96d77c8467f657fe5bd7d126e0f
Author: Dave Airlie <airlied@...ux.ie>
Date:   Sun Mar 19 18:56:12 2006 +1100

    drm: fixup PCI DMA support

I haven't found anything to say if we are meant to be reserving the
pages or not. So I bring it to your attention, asking for help.

Thanks,
-Chris