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Subject: Re: [PATCH 09/16] page-flags: define PG_reserved behavior on compound
 pages
To:     Chris Wilson <chris@chris-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Mel Gorman <mgorman@suse.de>, 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
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From:   David Hildenbrand <david@redhat.com>
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Date:   Mon, 3 Feb 2020 18:10:49 +0100
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On 03.02.20 16:24, Chris Wilson wrote:
> Quoting Kirill A. Shutemov (2020-02-03 15:18:44)
>> On Fri, Jan 31, 2020 at 03:24:12PM +0000, Chris Wilson wrote:
>>> 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 page=
s.
>>>> 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 physic=
ally
>>> contiguous) and mark each page as Reserved.
>>>
>>>         dmah->vaddr =3D 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 =3D (unsigned long)dmah->vaddr, sz =3D size;
>>>              sz > 0; addr +=3D PAGE_SIZE, sz -=3D 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.
>>
>> I don't see a real reason for these pages to be reserved. But I might =
be
>> wrong here.
>>
>> I tried to look around: other users (infiniband/ethernet) of
>> dma_alloc_coherent(__GFP_COMP) don't mess with PG_reserved.
>>
>> Could you try to drop it from DRM?
>=20
> That is the current plan. So long as there is nothing magical about
> either the __GFP_COMP or SetPageReserved, we should be able to drop the=
m
> without any functional change. Only 2 very old bits of HW (r128, ancien=
t
> i915) depend on this routine, and i915 seems, touch wood, quite happy
> with a plain dma_alloc_coherent().

I documented a while ago in include/linux/page-flags.h

"
Pages marked as PG_reserved include:
[...]
MMIO/DMA pages. Some architectures don't allow to ioremap pages that are
not marked PG_reserved (as they might be in use by somebody else who
does not respect the caching strategy).
"

I also removed a bunch of users back then (and even had a patch to
remove this code here), but for this code I think I came to the
conclusion that it might be relevant for some archs.

git grep -o PageReserved | grep ioremap
arch/mips/mm/ioremap.c:PageReserved
arch/nios2/mm/ioremap.c:PageReserved
arch/parisc/mm/ioremap.c:PageReserved
arch/x86/mm/ioremap.c:PageReserved

It would be good to clarify if this here is actually needed (and in
addition the same pattern in other driver/ paths) and eventually update
the documentation.

--=20
Thanks,

David / dhildenb