From: Christoph Hellwig Subject: Re: [PATCH 4/6] mm/hmm: add output flag for compound page mapping Date: Fri, 8 May 2020 21:51:30 +0200 Message-ID: <20200508195129.GA19740@lst.de> References: <20200508192009.15302-1-rcampbell@nvidia.com> <20200508192009.15302-5-rcampbell@nvidia.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200508192009.15302-5-rcampbell-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Ralph Campbell Cc: nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org, linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kselftest-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Jerome Glisse , John Hubbard , Christoph Hellwig , Jason Gunthorpe , Ben Skeggs , Andrew Morton , Shuah Khan , willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org List-Id: nouveau.vger.kernel.org On Fri, May 08, 2020 at 12:20:07PM -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. > Add 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. Why do you need the flag? The caller should be able to just use page_size() (or willys new thp_size helper).