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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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