Re: [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2)

[Vlastimil Babka <vbabka@suse.cz>]

On 10/16/18 9:43 PM, Joel Fernandes wrote:
> On Tue, Oct 16, 2018 at 01:29:52PM +0200, Vlastimil Babka wrote:
>> On 10/16/18 12:33 AM, Joel Fernandes wrote:
>>> On Mon, Oct 15, 2018 at 02:42:09AM -0700, Christoph Hellwig wrote:
>>>> On Fri, Oct 12, 2018 at 06:31:58PM -0700, Joel Fernandes (Google) wrote:
>>>>> Android needs to mremap large regions of memory during memory management
>>>>> related operations.
>>>>
>>>> Just curious: why?
>>>
>>> In Android we have a requirement of moving a large (up to a GB now, but may
>>> grow bigger in future) memory range from one location to another.
>>
>> I think Christoph's "why?" was about the requirement, not why it hurts
>> applications. I admit I'm now also curious :)
> 
> This issue was discovered when we wanted to be able to move the physical
> pages of a memory range to another location quickly so that, after the
> application threads are resumed, UFFDIO_REGISTER_MODE_MISSING userfaultfd
> faults can be received on the original memory range. The actual operations
> performed on the memory range are beyond the scope of this discussion. The
> user threads continue to refer to the old address which will now fault. The
> reason we want retain the old memory range and receives faults there is to
> avoid the need to fix the addresses all over the address space of the threads
> after we finish with performing operations on them in the fault handlers, so
> we mremap it and receive faults at the old addresses.
> 
> Does that answer your question?

Yes, interesting, thanks!

Vlastimil

> thanks,
> 
> - Joel
>