linux-kernel - Re: [PATCH 4/6] mm/hmm: add output flag for compound page mapping

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Message-ID: <20200508195129.GA19740@lst.de>
Date:   Fri, 8 May 2020 21:51:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     nouveau@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...uxfoundation.org>, willy@...radead.org
Subject: Re: [PATCH 4/6] mm/hmm: add output flag for compound page mapping

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).