From: Jason Gunthorpe <jgg@mellanox.com> To: Matthew Wilcox <willy@infradead.org> Cc: Ralph Campbell <rcampbell@nvidia.com>, linux-rdma@vger.kernel.org, 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>, Christoph Hellwig <hch@lst.de>, Andrew Morton <akpm@linux-foundation.org>, Shuah Khan <shuah@kernel.org>, Ben Skeggs <bskeggs@redhat.com> Subject: Re: [PATCH v2 2/5] mm/hmm: add output flags for PMD/PUD page mapping Date: Tue, 30 Jun 2020 19:20:55 -0300 [thread overview] Message-ID: <20200630222055.GM23821@mellanox.com> (raw) In-Reply-To: <20200630212343.GP25523@casper.infradead.org> On Tue, Jun 30, 2020 at 10:23:43PM +0100, Matthew Wilcox wrote: > On Tue, Jun 30, 2020 at 12:57:34PM -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 a > > compound page is mapped by the CPU using a larger page size. Without > > this information, the caller can't safely use a large device PTE to map > > the compound page because the CPU might be using smaller PTEs with > > different read/write permissions. > > > > Add two new output flags to indicate the mapping size (PMD or PUD sized) > > so that callers know the pages are being mapped with consistent permissions > > and a large device page table mapping can be used if one is available. > > The problem I have with this is that PTE/PMD/PUD are not the only choices > for how the CPU might choose to map something. For example, ARM has > the ability to map 64kB pages using 16 consecutive page table entries > (marked specially so the CPU knows to use a single TLB entry for the > 64kB range). Some other CPUs have similar capabilities. Sure, but at the moment this is the only thing hmm_range_fault() is able to detect and set.. > I'd rather you encoded the order of the mapping in the flags (eg a > number between 0 and 31) so that we have the flexibility in the future > to describe how memory is mapped. How about some hmm_get_mapping_order() API, we can keep the flags that match the implementation but the driver facing API will see something more general? Jason
WARNING: multiple messages have this Message-ID (diff)
From: Jason Gunthorpe <jgg-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org> To: Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> Cc: Ralph Campbell <rcampbell-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>, linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org, linux-kselftest-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Jerome Glisse <jglisse-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, John Hubbard <jhubbard-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>, Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Shuah Khan <shuah-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Ben Skeggs <bskeggs-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Subject: Re: [PATCH v2 2/5] mm/hmm: add output flags for PMD/PUD page mapping Date: Tue, 30 Jun 2020 19:20:55 -0300 [thread overview] Message-ID: <20200630222055.GM23821@mellanox.com> (raw) In-Reply-To: <20200630212343.GP25523-FZi0V3Vbi30CUdFEqe4BF2D2FQJk+8+b@public.gmane.org> On Tue, Jun 30, 2020 at 10:23:43PM +0100, Matthew Wilcox wrote: > On Tue, Jun 30, 2020 at 12:57:34PM -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 a > > compound page is mapped by the CPU using a larger page size. Without > > this information, the caller can't safely use a large device PTE to map > > the compound page because the CPU might be using smaller PTEs with > > different read/write permissions. > > > > Add two new output flags to indicate the mapping size (PMD or PUD sized) > > so that callers know the pages are being mapped with consistent permissions > > and a large device page table mapping can be used if one is available. > > The problem I have with this is that PTE/PMD/PUD are not the only choices > for how the CPU might choose to map something. For example, ARM has > the ability to map 64kB pages using 16 consecutive page table entries > (marked specially so the CPU knows to use a single TLB entry for the > 64kB range). Some other CPUs have similar capabilities. Sure, but at the moment this is the only thing hmm_range_fault() is able to detect and set.. > I'd rather you encoded the order of the mapping in the flags (eg a > number between 0 and 31) so that we have the flexibility in the future > to describe how memory is mapped. How about some hmm_get_mapping_order() API, we can keep the flags that match the implementation but the driver facing API will see something more general? Jason
next prev parent reply other threads:[~2020-06-30 22:21 UTC|newest] Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-06-30 19:57 [PATCH v2 0/5] mm/hmm/nouveau: add PMD system memory mapping Ralph Campbell 2020-06-30 19:57 ` Ralph Campbell 2020-06-30 19:57 ` [PATCH v2 1/5] nouveau/hmm: fault one page at a time Ralph Campbell 2020-06-30 19:57 ` Ralph Campbell 2020-06-30 19:57 ` [PATCH v2 2/5] mm/hmm: add output flags for PMD/PUD page mapping Ralph Campbell 2020-06-30 19:57 ` Ralph Campbell 2020-06-30 21:23 ` Matthew Wilcox 2020-06-30 21:23 ` Matthew Wilcox 2020-06-30 22:20 ` Jason Gunthorpe [this message] 2020-06-30 22:20 ` Jason Gunthorpe 2020-06-30 19:57 ` [PATCH v2 3/5] nouveau: fix mapping 2MB sysmem pages Ralph Campbell 2020-06-30 19:57 ` Ralph Campbell 2020-06-30 19:57 ` [PATCH v2 4/5] nouveau/hmm: support mapping large " Ralph Campbell 2020-06-30 19:57 ` Ralph Campbell 2020-06-30 19:57 ` [PATCH v2 5/5] hmm: add tests for HMM_PFN_PMD flag Ralph Campbell 2020-06-30 19:57 ` Ralph Campbell
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