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

[Chris Wilson]

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().
-Chris