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

[David Hildenbrand]

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 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@xxxxxxxx>
>>> 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?
> 
> 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 them
> without any functional change. Only 2 very old bits of HW (r128, ancient
> 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.

-- 
Thanks,

David / dhildenb