linux-kernel - Re: [PATCH 0/6] nouveau/hmm: add support for mapping large pages

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Message-ID: <20200525134118.GA2536@ziepe.ca>
Date:   Mon, 25 May 2020 10:41:18 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
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>,
        Ben Skeggs <bskeggs@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...uxfoundation.org>
Subject: Re: [PATCH 0/6] nouveau/hmm: add support for mapping large pages

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.
> 
> Note that this series depends on a patch queued in Ben Skeggs' nouveau
> tree ("nouveau/hmm: map pages after migration") and the patches queued
> in Jason's HMM tree.
> There is also a patch outstanding ("nouveau/hmm: fix nouveau_dmem_chunk
> allocations") that is independent of the above and could be applied
> before or after.

Did Christoph and Matt's remarks get addressed here?

I think ODP could use something like this, currently it checks every
page to get back to the huge page size and this flag would optimze
that

Jason