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Date:   Mon, 26 Sep 2022 16:53:37 +0200
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Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
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To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-doc@vger.kernel.org, qemu-devel@nongnu.org,
        Jonathan Corbet <corbet@lwn.net>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
        Steven Price <steven.price@arm.com>,
        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>, luto@kernel.org,
        jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com,
        aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>, mhocko@suse.com,
        Muchun Song <songmuchun@bytedance.com>, wei.w.wang@intel.com
References: <20220915142913.2213336-1-chao.p.peng@linux.intel.com>
 <20220915142913.2213336-2-chao.p.peng@linux.intel.com>
 <d16284f5-3493-2892-38e6-f1fa5c10bdbb@redhat.com>
 <20220923005808.vfltoecttoatgw5o@box.shutemov.name>
 <f703e615-3b75-96a2-fb48-2fefd8a2069b@redhat.com>
 <20220926144854.dyiacztlpx4fkjs5@box.shutemov.name>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20220926144854.dyiacztlpx4fkjs5@box.shutemov.name>
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On 26.09.22 16:48, Kirill A. Shutemov wrote:
> On Mon, Sep 26, 2022 at 12:35:34PM +0200, David Hildenbrand wrote:
>> On 23.09.22 02:58, Kirill A . Shutemov wrote:
>>> On Mon, Sep 19, 2022 at 11:12:46AM +0200, David Hildenbrand wrote:
>>>>> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
>>>>> index 6325d1d0e90f..9d066be3d7e8 100644
>>>>> --- a/include/uapi/linux/magic.h
>>>>> +++ b/include/uapi/linux/magic.h
>>>>> @@ -101,5 +101,6 @@
>>>>>     #define DMA_BUF_MAGIC		0x444d4142	/* "DMAB" */
>>>>>     #define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
>>>>>     #define SECRETMEM_MAGIC		0x5345434d	/* "SECM" */
>>>>> +#define INACCESSIBLE_MAGIC	0x494e4143	/* "INAC" */
>>>>
>>>>
>>>> [...]
>>>>
>>>>> +
>>>>> +int inaccessible_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
>>>>> +			 int *order)
>>>>> +{
>>>>> +	struct inaccessible_data *data = file->f_mapping->private_data;
>>>>> +	struct file *memfd = data->memfd;
>>>>> +	struct page *page;
>>>>> +	int ret;
>>>>> +
>>>>> +	ret = shmem_getpage(file_inode(memfd), offset, &page, SGP_WRITE);
>>>>> +	if (ret)
>>>>> +		return ret;
>>>>> +
>>>>> +	*pfn = page_to_pfn_t(page);
>>>>> +	*order = thp_order(compound_head(page));
>>>>> +	SetPageUptodate(page);
>>>>> +	unlock_page(page);
>>>>> +
>>>>> +	return 0;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(inaccessible_get_pfn);
>>>>> +
>>>>> +void inaccessible_put_pfn(struct file *file, pfn_t pfn)
>>>>> +{
>>>>> +	struct page *page = pfn_t_to_page(pfn);
>>>>> +
>>>>> +	if (WARN_ON_ONCE(!page))
>>>>> +		return;
>>>>> +
>>>>> +	put_page(page);
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(inaccessible_put_pfn);
>>>>
>>>> Sorry, I missed your reply regarding get/put interface.
>>>>
>>>> https://lore.kernel.org/linux-mm/20220810092532.GD862421@chaop.bj.intel.com/
>>>>
>>>> "We have a design assumption that somedays this can even support non-page
>>>> based backing stores."
>>>>
>>>> As long as there is no such user in sight (especially how to get the memfd
>>>> from even allocating such memory which will require bigger changes), I
>>>> prefer to keep it simple here and work on pages/folios. No need to
>>>> over-complicate it for now.
>>>
>>> Sean, Paolo , what is your take on this? Do you have conrete use case of
>>> pageless backend for the mechanism in sight? Maybe DAX?
>>
>> The problem I'm having with this is how to actually get such memory into the
>> memory backend (that triggers notifiers) and what the semantics are at all
>> with memory that is not managed by the buddy.
>>
>> memfd with fixed PFNs doesn't make too much sense.
> 
> What do you mean by "fixed PFN". It is as fixed as struct page/folio, no?
> PFN covers more possible backends.

For DAX, you usually bypass the buddy and map /dev/mem or a devdax. In 
contrast to ordinary memfd that allocates memory via the buddy. That's 
the difference I see -- and I wonder how it could work.

> 
>> When using DAX, what happens with the shared <->private conversion? Which
>> "type" is supposed to use dax, which not?
>>
>> In other word, I'm missing too many details on the bigger picture of how
>> this would work at all to see why it makes sense right now to prepare for
>> that.
> 
> IIUC, KVM doesn't really care about pages or folios. They need PFN to
> populate SEPT. Returning page/folio would make KVM do additional steps to
> extract PFN and one more place to have a bug.

Fair enough. Smells KVM specific, though.

-- 
Thanks,

David / dhildenb