Date:   Thu, 19 Aug 2021 13:00:34 -0500
From:   "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
To:     Ralph Campbell <rcampbell@...dia.com>,
        Felix Kuehling <felix.kuehling@....com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jgg@...dia.com, jglisse@...hat.com
Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page
 refcount


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@...dia.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@...dia.com>
>>>> Signed-off-by: Alex Sierra <alex.sierra@....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&amp;data=04%7C01%7Calex.sierra%40amd.com%7Cd2bd2d4fbf764528540908d9627e5dcd%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637649117156919772%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=P7FxYm%2BkJaCkMFa3OHtuKrPOn7SvytFRmYQdIzq7rN4%3D&amp;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&amp;data=04%7C01%7Calex.sierra%40amd.com%7Cd2bd2d4fbf764528540908d9627e5dcd%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637649117156919772%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=IXe8HP2s8x5OdJdERBkGOYJCQk3iqCu5AYkwpDL8zec%3D&amp;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.
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.

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. However,
I still need test to validate this. For migrate.c is used in 
remove_migration_pte and
migrate_vma_insert_page, however these are specific conditions for 
private device type
Thanks for raise these questions, I think we're getting close.

Regards,
Alex S.
>
> I still think this patch needs an ACK from a FS/DAX maintainer.
>

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