linux-kernel - Re: [PATCH v3 3/6] mm: support THP migration to device private memory

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Date:   Wed, 2 Dec 2020 11:08:54 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Ralph Campbell <rcampbell@...dia.com>,
        Christoph Hellwig <hch@....de>, linux-mm@...ck.org,
        nouveau@...ts.freedesktop.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Alistair Popple <apopple@...dia.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Zi Yan <ziy@...dia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roger Pau Monne <roger.pau@...rix.com>
Subject: Re: [PATCH v3 3/6] mm: support THP migration to device private
 memory

On Fri, Nov 20, 2020 at 04:01:33PM -0400, Jason Gunthorpe wrote:
> On Wed, Nov 11, 2020 at 03:38:42PM -0800, Ralph Campbell wrote:
> 
> > MEMORY_DEVICE_GENERIC:
> > Struct pages are created in dev_dax_probe() and represent non-volatile memory.
> > The device can be mmap()'ed which calls dax_mmap() which sets
> > vma->vm_flags | VM_HUGEPAGE.
> > A CPU page fault will result in a PTE, PMD, or PUD sized page
> > (but not compound) to be inserted by vmf_insert_mixed() which will call either
> > insert_pfn() or insert_page().
> > Neither insert_pfn() nor insert_page() increments the page reference
> > count.
> 
> But why was this done? It seems very strange to put a pfn with a
> struct page into a VMA and then deliberately not take the refcount for
> the duration of that pfn being in the VMA?
> 
> What prevents memunmap_pages() from progressing while VMAs still point
> at the memory?

Agreed.  Adding Roger who added MEMORY_DEVICE_GENERIC and the only
user.

> > I think just leaving the page reference count at one is better than trying
> > to use the mmu_interval_notifier or changing vmf_insert_mixed() and
> > invalidations of pfn_t_devmap(pfn) to adjust the page reference count.
> 
> Why so? The entire point of getting struct page's for this stuff was
> to be able to follow the struct page flow. I never did learn a reason
> why there is devmap stuff all over the place in the page table code...

Exactly.