Re: [PATCH 00/11] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON

[David Hildenbrand <david@redhat.com>]

On 20.06.25 21:28, Lorenzo Stoakes wrote:
> On Fri, Jun 20, 2025 at 07:59:17PM +0100, Pedro Falcato wrote:
>> On Tue, Jun 17, 2025 at 11:57:11AM +0100, Lorenzo Stoakes wrote:
>>> On Tue, Jun 17, 2025 at 10:45:53AM +0200, David Hildenbrand wrote:
>>>> mremap() is already an expensive operation ... so I think we need a pretty
>>>> convincing case to make this configurable by the user at all for each
>>>> individual mremap() invocation.
>>>
>>> My measurements suggest, unless you hit a very unfortunate case of -huge
>>> faulted in range all mapped PTE- that the work involved is not all that
>>> much more substantial in terms of order of magnitude than a normal mremap()
>>> operation.
>>>
>>
>> Could you share your measurements and/or post them on the cover letter for the
>> next version?
> 
> Yeah am going to experiment nad gather some data for the next respin and see
> what might be possible.
> 
> I will present this kind of data then.
> 
>>
>> If indeed it makes no practical difference, maybe we could try to enable it by
>> default and see what happens...
> 
> Well it makes a difference, but the question is how much it matters (we have to
> traverse every single PTE for faulted-in memory vs. if we move page tables we
> can potentially move at PMD granularity saving 512 traversals, but if the folios
> are large then we're not really slower...).
> 
> I have some ideas... :)

As a first step, we could have some global way to enable/disable the 
optimization system-wide. We could then learn if there is really any 
workload that notices the change, while still having a way to revert to 
the old behavior on affected systems easily.

Just a thought, I still hope we can avoid all that. Again, mremap() is 
not really known for being a very efficient operation.

> 
>>
>> Or: separate but maybe awful idea, but if the problem is the number of VMAs
>> maybe we could try harder based on the map count? i.e if
>> map_count > (max_map_count / 2), try to relocate anon.
> 
> Interesting, though that'd make some things randomly merge and other stuff not,
> and you really have to consistently do this stuff to make things mergeable.

Yes, I'd prefer if we can make it more predictable.

(Of course, the VMA region size could also be used as an input to a 
policy. e.g., small move -> much fragmentation -> merge, large move -> 
less fragmentation -> don't care. Knowing about the use cases that use 
mremap() of anon memory and how they might be affected could be very 
valuable. Maybe it's mostly moving a handful of pages where we most care 
about this optimization?).


-- 
Cheers,

David / dhildenb