[PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page refcount

Fri Aug 20 07:17:41 UTC 2021

On Thu, Aug 19, 2021 at 11:00 AM Sierra Guiza, Alejandro (Alex)
<alex.sierra at amd.com> wrote:
>
>
> On 8/18/2021 2:28 PM, Ralph Campbell wrote:
> > On 8/17/21 5:35 PM, Felix Kuehling wrote:
> >> Am 2021-08-17 um 8:01 p.m. schrieb Ralph Campbell:
> >>> On 8/12/21 11:31 PM, Alex Sierra wrote:
> >>>> From: Ralph Campbell <rcampbell at nvidia.com>
> >>>>
> >>>> ZONE_DEVICE struct pages have an extra reference count that
> >>>> complicates the
> >>>> code for put_page() and several places in the kernel that need to
> >>>> check the
> >>>> reference count to see that a page is not being used (gup, compaction,
> >>>> migration, etc.). Clean up the code so the reference count doesn't
> >>>> need to
> >>>> be treated specially for ZONE_DEVICE.
> >>>>
> >>>> v2:
> >>>> AS: merged this patch in linux 5.11 version
> >>>>
> >>>> v5:
> >>>> AS: add condition at try_grab_page to check for the zone device type,
> >>>> while
> >>>> page ref counter is checked less/equal to zero. In case of device
> >>>> zone, pages
> >>>> ref counter are initialized to zero.
> >>>>
> >>>> Signed-off-by: Ralph Campbell <rcampbell at nvidia.com>
> >>>> Signed-off-by: Alex Sierra <alex.sierra at amd.com>
> >>>> ---
> >>>>    arch/powerpc/kvm/book3s_hv_uvmem.c     |  2 +-
> >>>>    drivers/gpu/drm/nouveau/nouveau_dmem.c |  2 +-
> >>>>    fs/dax.c                               |  4 +-
> >>>>    include/linux/dax.h                    |  2 +-
> >>>>    include/linux/memremap.h               |  7 +--
> >>>>    include/linux/mm.h                     | 13 +----
> >>>>    lib/test_hmm.c                         |  2 +-
> >>>>    mm/internal.h                          |  8 +++
> >>>>    mm/memremap.c                          | 68
> >>>> +++++++-------------------
> >>>>    mm/migrate.c                           |  5 --
> >>>>    mm/page_alloc.c                        |  3 ++
> >>>>    mm/swap.c                              | 45 ++---------------
> >>>>    12 files changed, 46 insertions(+), 115 deletions(-)
> >>>>
> >>> I haven't seen a response to the issues I raised back at v3 of this
> >>> series.
> >>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flinux-mm%2F4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc%40nvidia.com%2F&data=04%7C01%7Calex.sierra%40amd.com%7Cd2bd2d4fbf764528540908d9627e5dcd%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637649117156919772%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=P7FxYm%2BkJaCkMFa3OHtuKrPOn7SvytFRmYQdIzq7rN4%3D&reserved=0
> >>>
> >>>
> >>>
> >>> Did I miss something?
> >> I think part of the response was that we did more testing. Alex added
> >> support for DEVICE_GENERIC pages to test_hmm and he ran DAX tests
> >> recommended by Theodore Tso. In that testing he ran into a WARN_ON_ONCE
> >> about a zero page refcount in try_get_page. The fix is in the latest
> >> version of patch 2. But it's already obsolete because John Hubbard is
> >> about to remove that function altogether.
> >>
> >> I think the issues you raised were more uncertainty than known bugs. It
> >> seems the fact that you can have DAX pages with 0 refcount is a feature
> >> more than a bug.
> >>
> >> Regards,
> >>    Felix
> >
> > Did you test on a system without CONFIG_ARCH_HAS_PTE_SPECIAL defined?
> > In that case, mmap() of a DAX device will call insert_page() which calls
> > get_page() which would trigger VM_BUG_ON_PAGE().
> >
> > I can believe it is OK for PTE_SPECIAL page table entries to have no
> > struct page or that MEMORY_DEVICE_GENERIC struct pages be mapped with
> > a zero reference count using insert_pfn().
> Hi Ralph,
> We have tried the DAX tests with and without CONFIG_ARCH_HAS_PTE_SPECIAL
> defined.
> Apparently none of the tests touches that condition for a DAX device. Of
> course,
> that doesn't mean it could happen.
>
> Regards,
> Alex S.
>
> >
> >
> > I find it hard to believe that other MM developers don't see an issue
> > with a struct page with refcount == 0 and mapcount == 1.
> >
> > I don't see where init_page_count() is being called for the
> > MEMORY_DEVICE_GENERIC or MEMORY_DEVICE_PRIVATE struct pages the AMD
> > driver allocates and passes to migrate_vma_setup().
> > Looks like svm_migrate_get_vram_page() needs to call init_page_count()
> > instead of get_page(). (I'm looking at branch
> > origin/alexsierrag/device_generic
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FRadeonOpenCompute%2FROCK-Kernel-Driver.git&data=04%7C01%7Calex.sierra%40amd.com%7Cd2bd2d4fbf764528540908d9627e5dcd%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637649117156919772%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=IXe8HP2s8x5OdJdERBkGOYJCQk3iqCu5AYkwpDL8zec%3D&reserved=0)
> Yes, you're right. My bad. Thanks for catching this up. I didn't realize
> I was missing
> to define CONFIG_DEBUG_VM on my build. Therefore this BUG was never caught.
> It worked after I replaced get_pages by init_page_count at
> svm_migrate_get_vram_page. However, I don't think this is the best way
> to fix it.

You definitly don't want to do that. reiniting the page refcounter is
wrong it should be done once. Nouveau is not a good example here.

> Ideally, get_pages call should work for device pages with ref count
> equal to 0
> too. Otherwise, we could overwrite refcounter if someone else is
> grabbing the page
> concurrently.
> I was thinking to add a special condition in get_pages for dev pages.
> This could
> also fix the insert_page -> get_page call from a DAX device.

What is the issue here exactly ?

>
> Regards,
> Alex S.
> >
> >
> > Also, what about the other places where is_device_private_page() is
> > called?
> > Don't they need to be updated to call is_device_page() instead?
> > One of my goals for this patch was to remove special casing reference
> > counts
> > for ZONE_DEVICE pages in rmap.c, etc.
> Correct, is_device_private_page is still used in rmap, memcontrol and
> migrate.c files
> Looks like rmap and memcontrol should be replaced by is_device_page
> function.

No you do not want to do that. The private case is special case for a reason.

Jerome