From: Christoph Hellwig <hch@lst.de>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>,
Christoph Hellwig <hch@lst.de>,
linux-mm@kvack.org, nouveau@lists.freedesktop.org,
linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
Jerome Glisse <jglisse@redhat.com>,
John Hubbard <jhubbard@nvidia.com>,
Alistair Popple <apopple@nvidia.com>,
Bharata B Rao <bharata@linux.ibm.com>, Zi Yan <ziy@nvidia.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Yang Shi <yang.shi@linux.alibaba.com>,
Ben Skeggs <bskeggs@redhat.com>, Shuah Khan <shuah@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH v3 3/6] mm: support THP migration to device private memory
Date: Wed, 2 Dec 2020 11:08:54 +0100 [thread overview]
Message-ID: <20201202100854.GB7597@lst.de> (raw)
In-Reply-To: <20201120200133.GH917484@nvidia.com>
On Fri, Nov 20, 2020 at 04:01:33PM -0400, Jason Gunthorpe wrote:
> On Wed, Nov 11, 2020 at 03:38:42PM -0800, Ralph Campbell wrote:
>
> > MEMORY_DEVICE_GENERIC:
> > Struct pages are created in dev_dax_probe() and represent non-volatile memory.
> > The device can be mmap()'ed which calls dax_mmap() which sets
> > vma->vm_flags | VM_HUGEPAGE.
> > A CPU page fault will result in a PTE, PMD, or PUD sized page
> > (but not compound) to be inserted by vmf_insert_mixed() which will call either
> > insert_pfn() or insert_page().
> > Neither insert_pfn() nor insert_page() increments the page reference
> > count.
>
> But why was this done? It seems very strange to put a pfn with a
> struct page into a VMA and then deliberately not take the refcount for
> the duration of that pfn being in the VMA?
>
> What prevents memunmap_pages() from progressing while VMAs still point
> at the memory?
Agreed. Adding Roger who added MEMORY_DEVICE_GENERIC and the only
user.
> > I think just leaving the page reference count at one is better than trying
> > to use the mmu_interval_notifier or changing vmf_insert_mixed() and
> > invalidations of pfn_t_devmap(pfn) to adjust the page reference count.
>
> Why so? The entire point of getting struct page's for this stuff was
> to be able to follow the struct page flow. I never did learn a reason
> why there is devmap stuff all over the place in the page table code...
Exactly.
next prev parent reply other threads:[~2020-12-02 10:09 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-06 0:51 [PATCH v3 0/6] mm/hmm/nouveau: add THP migration to migrate_vma_* Ralph Campbell
2020-11-06 0:51 ` [PATCH v3 1/6] mm/thp: add prep_transhuge_device_private_page() Ralph Campbell
2020-11-06 7:55 ` Christoph Hellwig
2020-11-06 20:56 ` Ralph Campbell
2020-11-06 12:14 ` Matthew Wilcox
2020-11-06 20:34 ` Ralph Campbell
2020-11-06 0:51 ` [PATCH v3 2/6] mm/migrate: move migrate_vma_collect_skip() Ralph Campbell
2020-11-06 7:56 ` Christoph Hellwig
2020-11-06 7:57 ` Christoph Hellwig
2020-11-06 0:51 ` [PATCH v3 3/6] mm: support THP migration to device private memory Ralph Campbell
2020-11-06 8:03 ` Christoph Hellwig
2020-11-06 21:26 ` Ralph Campbell
2020-11-09 9:14 ` Christoph Hellwig
2020-11-09 21:34 ` Ralph Campbell
2020-11-11 23:38 ` Ralph Campbell
2020-11-20 20:01 ` Jason Gunthorpe
2020-12-02 10:08 ` Christoph Hellwig [this message]
2020-12-05 8:22 ` Roger Pau Monné
2020-12-02 10:14 ` Christoph Hellwig
2020-12-02 18:01 ` Logan Gunthorpe
2020-11-06 0:51 ` [PATCH v3 4/6] mm/thp: add THP allocation helper Ralph Campbell
2020-11-06 8:01 ` Christoph Hellwig
2020-11-06 21:09 ` Ralph Campbell
2020-11-06 0:51 ` [PATCH v3 5/6] mm/hmm/test: add self tests for THP migration Ralph Campbell
2020-11-06 0:51 ` [PATCH v3 6/6] nouveau: support THP migration to private memory Ralph Campbell
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