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

[David Hildenbrand <david@redhat.com>]

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