Re: [PATCH 09/16] page-flags: define PG_reserved behavior on compound pages

From: Chris Wilson <chris@chris-wilson.co.uk>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
	Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
	Christoph Lameter <cl@gentwo.org>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Steve Capper <steve.capper@linaro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@suse.cz>,
	Jerome Marchand <jmarchan@redhat.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 09/16] page-flags: define PG_reserved behavior on compound pages
Date: Fri, 31 Jan 2020 15:24:12 +0000	[thread overview]
Message-ID: <158048425224.2430.4905670949721797624@skylake-alporthouse-com> (raw)
In-Reply-To: <1426784902-125149-10-git-send-email-kirill.shutemov@linux.intel.com>

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@linux.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