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

--
Kirill A. Shutemov

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