From: Matthew Wilcox <willy@infradead.org>
To: Ralph Campbell <rcampbell@nvidia.com>
Cc: nouveau@lists.freedesktop.org, linux-rdma@vger.kernel.org,
linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
linux-kernel@vger.kernel.org, Jerome Glisse <jglisse@redhat.com>,
John Hubbard <jhubbard@nvidia.com>,
Christoph Hellwig <hch@lst.de>,
Jason Gunthorpe <jgg@mellanox.com>,
Ben Skeggs <bskeggs@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH 0/6] nouveau/hmm: add support for mapping large pages
Date: Fri, 8 May 2020 20:17:26 -0700 [thread overview]
Message-ID: <20200509031726.GT16070@bombadil.infradead.org> (raw)
In-Reply-To: <72422dca-e025-002a-4748-addfb392ffc4@nvidia.com>
On Fri, May 08, 2020 at 01:17:55PM -0700, Ralph Campbell wrote:
> On 5/8/20 12:59 PM, Matthew Wilcox wrote:
> > On Fri, May 08, 2020 at 12:20:03PM -0700, Ralph Campbell wrote:
> > > hmm_range_fault() returns an array of page frame numbers and flags for
> > > how the pages are mapped in the requested process' page tables. The PFN
> > > can be used to get the struct page with hmm_pfn_to_page() and the page size
> > > order can be determined with compound_order(page) but if the page is larger
> > > than order 0 (PAGE_SIZE), there is no indication that the page is mapped
> > > using a larger page size. To be fully general, hmm_range_fault() would need
> > > to return the mapping size to handle cases like a 1GB compound page being
> > > mapped with 2MB PMD entries. However, the most common case is the mapping
> > > size the same as the underlying compound page size.
> > > This series adds a new output flag to indicate this so that callers know it
> > > is safe to use a large device page table mapping if one is available.
> > > Nouveau and the HMM tests are updated to use the new flag.
> >
> > This explanation doesn't make any sense. It doesn't matter how somebody
> > else has it mapped; if it's a PMD-sized page, you can map it with a
> > 2MB mapping.
>
> Sure, the I/O will work OK, but is it safe?
> Copy on write isn't an issue? splitting a PMD in one process due to
> mprotect of a shared page will cause other process' page tables to be split
> the same way?
Are you saying that if you call this function on an address range of a
process which has done COW of a single page in the middle of a THP,
you want to return with this flag clear, but if the THP is still intact,
you want to set this flag?
> Recall that these are system memory pages that could be THPs, shmem, hugetlbfs,
> mmap shared file pages, etc.
next prev parent reply other threads:[~2020-05-09 3:17 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-08 19:20 [PATCH 0/6] nouveau/hmm: add support for mapping large pages Ralph Campbell
2020-05-08 19:20 ` [PATCH 1/6] nouveau/hmm: map pages after migration Ralph Campbell
2020-05-08 19:20 ` [PATCH 2/6] nouveau: make nvkm_vmm_ctor() and nvkm_mmu_ptp_get() static Ralph Campbell
2020-05-08 19:20 ` [PATCH 3/6] nouveau/hmm: fault one page at a time Ralph Campbell
2020-05-08 19:20 ` [PATCH 4/6] mm/hmm: add output flag for compound page mapping Ralph Campbell
2020-05-08 19:51 ` Christoph Hellwig
2020-05-08 20:06 ` Ralph Campbell
2020-05-26 22:29 ` Zi Yan
2020-05-26 22:47 ` Ralph Campbell
2020-05-08 19:20 ` [PATCH 5/6] nouveau/hmm: support mapping large sysmem pages Ralph Campbell
2020-05-08 19:20 ` [PATCH 6/6] hmm: add tests for HMM_PFN_COMPOUND flag Ralph Campbell
2020-05-08 19:59 ` [PATCH 0/6] nouveau/hmm: add support for mapping large pages Matthew Wilcox
2020-05-08 20:17 ` Ralph Campbell
2020-05-09 3:17 ` Matthew Wilcox [this message]
2020-05-11 17:07 ` Ralph Campbell
2020-05-25 13:41 ` Jason Gunthorpe
2020-05-26 17:32 ` Ralph Campbell
2020-05-29 23:24 ` Jason Gunthorpe
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