From: Ralph Campbell <rcampbell@nvidia.com>
To: Alex Sierra <alex.sierra@amd.com>, <akpm@linux-foundation.org>,
<Felix.Kuehling@amd.com>, <linux-mm@kvack.org>,
<linux-ext4@vger.kernel.org>, <linux-xfs@vger.kernel.org>
Cc: <amd-gfx@lists.freedesktop.org>,
<dri-devel@lists.freedesktop.org>, <hch@lst.de>, <jgg@nvidia.com>,
<jglisse@redhat.com>
Subject: Re: [PATCH v3 2/8] mm: remove extra ZONE_DEVICE struct page refcount
Date: Thu, 17 Jun 2021 12:16:26 -0700 [thread overview]
Message-ID: <7163dbb6-67b5-6eef-5772-500fd2107e5c@nvidia.com> (raw)
In-Reply-To: <20210617151705.15367-3-alex.sierra@amd.com>
On 6/17/21 8:16 AM, Alex Sierra wrote:
> From: Ralph Campbell <rcampbell@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
>
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Signed-off-by: Alex Sierra <alex.sierra@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 | 44 -----------------
> 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, 45 insertions(+), 147 deletions(-)
>
I think it is great that you are picking this up and trying to revive it.
However, I have a number of concerns about how it affects existing ZONE_DEVICE
MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_FS_DAX users and I don't see this
addressing them. For example, dev_dax_probe() allocates MEMORY_DEVICE_GENERIC
struct pages and then:
dev_dax_fault()
dev_dax_huge_fault()
__dev_dax_pte_fault()
vmf_insert_mixed()
which just inserts the PFN into the CPU page tables without increasing the page
refcount so it is zero (whereas it was one before). But using get_page() will
trigger VM_BUG_ON_PAGE() if it is enabled. There isn't any current notion of
free verses allocated for these struct pages. I suppose init_page_count()
could be called on all the struct pages in dev_dax_probe() to fix that though.
I'm even less clear about how to fix MEMORY_DEVICE_FS_DAX. File systems have clear
allocate and free states for backing storage but there are the complications with
the page cache references, etc. to consider. The >1 to 1 reference count seems to
be used to tell when a page is idle (no I/O, reclaim scanners) rather than free
(not allocated to any file) but I'm not 100% sure about that since I don't really
understand all the issues around why a file system needs to have a DAX mount option
besides knowing that the storage block size has to be a multiple of the page size.
next prev parent reply other threads:[~2021-06-17 19:16 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 15:16 [PATCH v3 0/8] Support DEVICE_GENERIC memory in migrate_vma_* Alex Sierra
2021-06-17 15:16 ` [PATCH v3 1/8] ext4/xfs: add page refcount helper Alex Sierra
2021-06-17 16:11 ` Darrick J. Wong
2021-06-17 23:52 ` Dave Chinner
2021-06-19 16:14 ` Sierra Guiza, Alejandro (Alex)
2021-06-19 18:31 ` Alex Sierra
2021-06-17 15:16 ` [PATCH v3 2/8] mm: remove extra ZONE_DEVICE struct page refcount Alex Sierra
2021-06-17 19:16 ` Ralph Campbell [this message]
2021-06-28 16:46 ` Felix Kuehling
2021-06-30 0:23 ` Ralph Campbell
2021-06-17 15:17 ` [PATCH v3 3/8] kernel: resource: lookup_resource as exported symbol Alex Sierra
2021-06-17 15:17 ` [PATCH v3 4/8] drm/amdkfd: add SPM support for SVM Alex Sierra
2021-06-17 15:17 ` [PATCH v3 5/8] drm/amdkfd: generic type as sys mem on migration to ram Alex Sierra
2021-06-17 15:17 ` [PATCH v3 6/8] include/linux/mm.h: helpers to check zone device generic type Alex Sierra
2021-06-17 15:17 ` [PATCH v3 7/8] mm: add generic type support to migrate_vma helpers Alex Sierra
2021-06-17 15:17 ` [PATCH v3 8/8] mm: call pgmap->ops->page_free for DEVICE_GENERIC pages Alex Sierra
2021-06-17 15:56 ` [PATCH v3 0/8] Support DEVICE_GENERIC memory in migrate_vma_* Sierra Guiza, Alejandro (Alex)
2021-06-20 14:14 ` Theodore Ts'o
2021-06-23 21:49 ` Felix Kuehling
2021-06-24 5:30 ` Christoph Hellwig
2021-06-24 15:08 ` Felix Kuehling
2021-07-16 15:07 ` Theodore Y. Ts'o
2021-07-16 22:14 ` Felix Kuehling
2021-07-17 19:54 ` Sierra Guiza, Alejandro (Alex)
2021-07-23 22:46 ` Sierra Guiza, Alejandro (Alex)
2021-07-30 19:02 ` Felix Kuehling
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