linux-ext4 - Re: [PATCH v6 02/13] mm: remove extra ZONE

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Date:   Tue, 17 Aug 2021 20:35:22 -0400
From:   Felix Kuehling <felix.kuehling@....com>
To:     Ralph Campbell <rcampbell@...dia.com>,
        Alex Sierra <alex.sierra@....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


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://lore.kernel.org/linux-mm/4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc@nvidia.com/
>
>
> 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