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Subject: Re: [PATCH 4/6] mm/hmm: add output flag for compound page mapping
To:     Christoph Hellwig <hch@lst.de>
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>,
        Jason Gunthorpe <jgg@mellanox.com>,
        "Ben Skeggs" <bskeggs@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Shuah Khan" <shuah@kernel.org>, <willy@infradead.org>
References: <20200508192009.15302-1-rcampbell@nvidia.com>
 <20200508192009.15302-5-rcampbell@nvidia.com> <20200508195129.GA19740@lst.de>
From:   Ralph Campbell <rcampbell@nvidia.com>
Message-ID: <1ec0511f-e6eb-3e11-ad6d-ad4e5b107464@nvidia.com>
Date:   Fri, 8 May 2020 13:06:55 -0700
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On 5/8/20 12:51 PM, Christoph Hellwig wrote:
> 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).
> 

The question is whether or not a large page can be mapped with smaller
page table entries with different permissions. If one process has a 2MB
page mapped with 4K PTEs with different read/write permissions, I don't think
it would be OK for a device to map the whole 2MB with write access enabled.
The flag is supposed to indicate that the whole page can be mapped by the
device with the indicated read/write permissions.