linux-kernel - Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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Message-ID: <0a99aa24-599c-cc60-b23b-b77887af3702@redhat.com>
Date:   Mon, 26 Sep 2022 16:53:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>, luto@...nel.org,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        aarcange@...hat.com, ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>, wei.w.wang@...el.com
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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