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Wed, 18 Aug 2021 12:28:31 -0700 Received: from HQMAIL111.nvidia.com (172.20.187.18) by HQMAIL105.nvidia.com (172.20.187.12) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Wed, 18 Aug 2021 19:28:31 +0000 Received: from rcampbell-test.nvidia.com (172.20.187.5) by mail.nvidia.com (172.20.187.18) with Microsoft SMTP Server (TLS) id 15.0.1497.2 via Frontend Transport; Wed, 18 Aug 2021 19:28:31 +0000 Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page refcount To: Felix Kuehling , Alex Sierra , , , , CC: , , , , References: <20210813063150.2938-1-alex.sierra@amd.com> <20210813063150.2938-3-alex.sierra@amd.com> <7b821150-af18-f786-e419-ec245b8cfb1e@nvidia.com> <393e9815-838d-5fe6-d6ab-bfe7b543fef6@amd.com> From: Ralph Campbell Message-ID: Date: Wed, 18 Aug 2021 12:28:30 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <393e9815-838d-5fe6-d6ab-bfe7b543fef6@amd.com> Content-Type: text/plain; charset="utf-8"; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Aug 2021 19:28:31.4867 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 0ca49a3a-3231-4322-e250-08d9627e5d0b X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.112.32];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: CO1NAM11FT031.eop-nam11.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR12MB1609 X-Rspamd-Queue-Id: 526671003FED Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=pt4FulzC; dmarc=pass (policy=quarantine) header.from=nvidia.com; spf=none (imf07.hostedemail.com: domain of rcampbell@nvidia.com has no SPF policy when checking 40.107.223.81) smtp.mailfrom=rcampbell@nvidia.com X-Rspamd-Server: rspam04 X-Stat-Signature: xhj5f51deo53whtuhcmgssu55dn6rzwc X-HE-Tag: 1629314913-66587 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 >>> >>> 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 >>> Signed-off-by: Alex Sierra >>> --- >>> =C2=A0 arch/powerpc/kvm/book3s_hv_uvmem.c=C2=A0=C2=A0=C2=A0=C2=A0 |=C2= =A0 2 +- >>> =C2=A0 drivers/gpu/drm/nouveau/nouveau_dmem.c |=C2=A0 2 +- >>> =C2=A0 fs/dax.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0 4 +- >>> =C2=A0 include/linux/dax.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 = |=C2=A0 2 +- >>> =C2=A0 include/linux/memremap.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0 7 +-- >>> =C2=A0 include/linux/mm.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= | 13 +---- >>> =C2=A0 lib/test_hmm.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0 |=C2=A0 2 +- >>> =C2=A0 mm/internal.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0 8 +++ >>> =C2=A0 mm/memremap.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 | 68 +++++++------------------- >>> =C2=A0 mm/migrate.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0 5 -- >>> =C2=A0 mm/page_alloc.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 |=C2=A0 3 ++ >>> =C2=A0 mm/swap.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | 45 ++--------------- >>> =C2=A0 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, > =C2=A0 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(). I find it hard to believe that other MM developers don't see an issue with a struct page with refcount =3D=3D 0 and mapcount =3D=3D 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_g= eneric https://github.com/RadeonOpenCompute/ROCK-Kernel-Driver.git) Also, what about the other places where is_device_private_page() is calle= d? 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 cou= nts for ZONE_DEVICE pages in rmap.c, etc. I still think this patch needs an ACK from a FS/DAX maintainer.